In Windows you have a C-drive. Everything labeled beyond that is with the following letter.

So your second drive is D, your DVD is E and if you put in a USB stick it becomes F and the following drive G. And so on and so forth.

But then, what and where are A and B-drives?

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    If you want to know something they can be used for now, one trick is to map them to USB drives. If you insert and remove a USB drive frequently, but want it to keep the same drive letter, you can map it to A or B. Windows attempts to reuse the last drive letter for any given device, and won't dynamically give out A or B, so you will be assured of getting the same drive letter next time.
    – Roger
    Commented Dec 16, 2013 at 22:38

20 Answers 20


Short Version: A: & B: are reserved by floppy disk drives, so C: is used by hard drives for backwards compatibility reasons.

Once upon a time, the early CP/M and IBM PC style computers had no hard drive. You had one floppy drive, and that was it. Unless you spent another $1k or so on a second floppy drive, then your system was smokin'! If you only had one drive it was common to boot from one disk, put in the other disk with your programs and data, then run the program. Once the program finished, the computer would request that you reinsert the boot disk so you could use the command line again. Copying data from one disk to the other was a series of

Please insert source disk into drive A:...
Please insert destination disk into drive A:...
Please insert source disk into drive A:...

By the time hard drives became cheap, the "expensive" computers typically had two floppy drives (one to boot and run common programs, one to save data and run specific programs). And so it was common for the motherboard hardware to support two floppy drives at fixed system addresses. Since it was built into the hardware, it was thought that building the same requirement into the OS was acceptable, and any hard drives added to the machine would start with disk C: and so forth.

During the transition from 5.25" disks (which were actually, physically floppy) to 3.5" disks (which were encased in a harder plastic shell) it was common to have both drives in one system, and again it was supported on the motherboard with hardware, and in the OS at fixed addresses. As very few systems ran out of drive letters, it was not thought to be important to consider making those drives re-assignable in the OS until much later when drives were abstracted along with addresses due to the plug'n'play standard.

A lot of software was developed since that time, and unfortunately much of it expected to see long-term storage on the C: drive. This includes the BIOS software that boots the computer. You can still attach two floppy drives, boot into DOS 6.1, and use it as you would have in the early 90's, with floppy drives A: and B:.

So largely the reason for starting the hard drive at C is for backwards compatibility. While the OS has abstracted data storage to some degree, it still treats A: and B: differently, in such a way that allows them to be removed from the system without altering the OS, caching them differently, and due to early viruses treating their boot sector with more caution than the hard drive's boot sector.

For Windows specifically, it's worth mentioning that you can use A: and B: as the names for volumes, be it a flash drive or an internal hard drive.

  • 35
    BIOS generally does not deal in drive letters; where did you get the idea that it did? (Maybe some "user friendly" ones make up letters that they think the OS will use, though...)
    – SamB
    Commented Jan 21, 2011 at 3:13
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    @SamB - The BIOS initializes both the floppy drives and the hard drives, and in some cases allow one to swap the position of the floppy drives (A: <--> B:). When booting from floppy some request, specifically, a floppy in drive A. But you are correct that this is merely UI icing so people aren't confused. If a user were to map the floppy to drive F:, for instance, they may be surprised to see the BIOS trying to boot from drive "A:" if they haven't set the bios to skip the floppy drives and boot the hard drive.
    – Adam Davis
    Commented Jan 21, 2011 at 4:06
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    Unless you had a NEC PC-98, in which case A: was usually your hard disk.
    – JasonTrue
    Commented Apr 25, 2012 at 21:41
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    "The early CP/M and IBM PC style computers had no hard drive. You had one floppy drive and that's the way it was and you liked it!" - fixed that for you. Commented Apr 26, 2012 at 22:45
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    @lunchmeat317 - *NIX systems were not relevant to the development of the PC-clone hardware pseudo-standards that evolved into today's PC. There were not enough users for the vendors to care at that time what the *NIX people did. Commented Dec 16, 2013 at 22:21

A: and B: were used for floppy disks.

Edit: Someone asked for pics so here's an 8", 5.25" and 3.5" floppy disk. (8" disks were not used as standard on IBM compatible personal computers.)

floppy disks

  • 55
    Perhaps it should be said that 5.25" disks weren’t typically bright orange. :) They were typically black, just like the 8" one in your picture, while the 3.5" ones came in a much greater variety of colours.
    – Timwi
    Commented Apr 27, 2012 at 16:38
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    And here's a 3" disk. Yes, 3", not 3.5". vintagecomputershop.co.uk/51-large/cf-2-3-disk-single.jpg
    – DrHyde
    Commented Oct 23, 2012 at 17:24
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    If I recall correctly, we weren't supposed to copy that floppy. youtube.com/watch?v=up863eQKGUI Commented Oct 19, 2013 at 20:09
  • 3.5" floppies were usually blue for DD and black for HD; later on, when DD floppies stopped existing, they made HD ones in just about every colour under the sun. Commented Oct 22, 2013 at 15:28
  • And here's a 3" disk. Yes, 3", not 3.5". Hmm, I hadn’t heard of that one. Granted it’s a narrow, single-purpose, proprietary format (only for Amstrad), but I’m still surprised I hadn’t heard of it because I thought I had seen it all, including an 5MB IBM 350.
    – Synetech
    Commented Apr 12, 2014 at 18:04

Less an answer, more of an anecdote. In this Microsoft article, it says:

"You can assign the letters C through Z to each drive on your computer. A and B are usually reserved for floppy disk drives, but if your computer does not have floppy disk drives, you can assign A and B to volumes."

So when I built a new computer recently with two internal drives, one for the OS and one for data, I thought, hey!, I'll make my data drive "A". I felt all rebellious until I discovered that Windows will not index drives lettered A or B. :( Took me quite a while to figure out what the problem was, but I found some other people who suffered the same issue when they used A or B for a drive. As soon as I assigned that drive a different letter, windows indexed the drive. So much for being rebellious. lol.

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    There was a bug in the Windows XP text-mode installer. It thought that a ZIP drive was a hard disk. A quirk of that got me a fully installed and working XP with the ZIP drive mapped to C:, my CDROM on D:, and the installed OS on E:. I remapped the ZIP and CDROM, leaving me with no C: at all. That machine shook out many faulty installers that assumed that Windows must be C:\Windows in its early years. My new Win7 box is "properly" installed on C: but I miss the quirks.
    – RBerteig
    Commented Jan 11, 2011 at 9:08
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    I assign A: B: to thumb drives and memory card drives; great way to relive the removable disks days.
    – icelava
    Commented Jan 12, 2011 at 11:09
  • You can assign A: and B: only to removable drives.
    – kinokijuf
    Commented Dec 10, 2011 at 21:19
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    @RBerteig: it's not a bug. It depends on what IDE channels you install them.
    – kinokijuf
    Commented Dec 10, 2011 at 21:21
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    A + B doesn't index? Seems like a letter already designed for SSDs twenty years ago
    – nixda
    Commented Aug 1, 2013 at 5:50

The two horizontal slots on the front of this PC are the A: and B: drives (5.25" floppies in this case). Note that you had to physically "mount" the disk by turning the lever on the front of the drive. Ahh, simpler times.

enter image description here

  • I forgot about those levers! I remember more than once the level only closing halfway and my disk making noises like it was getting crunched up!
    – TrojanName
    Commented Feb 3, 2012 at 16:07
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    @ken the software in this picture is P2 , now known as autocad :D
    – echolab
    Commented Jun 14, 2012 at 11:28
  • This is why you get error messages (even now) asking you to close the door on the drive - those levers are the doors. Commented Oct 22, 2013 at 15:30
  • @echolab - it is hard to tell exactly, but it looks to me more like a spreadsheet than a drafting/design software. Or, is it that Amstrad had a spreadsheet named "Autocad"?
    – ysap
    Commented Jan 8, 2014 at 17:27
  • In fact, we should be using "Mount" (With quotes) for today's computers ; at that time it was literal (in a sense)
    – Milind R
    Commented Feb 15, 2014 at 5:54

Drive letters A and B are reserved for floppy disk drives. However, if your computer does not have a floppy disk drive, you can assign these letters to removable drives.

Source: How to change drive letter assignments in Windows XP

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    Huh, you can also assign them to non-removable drives. I have a huge non-removable B:\ drive for Backups (previously A:\ for AcronisImages). (Argh at SO for having an unusable bolding mechanism...)
    – RomanSt
    Commented Jan 12, 2011 at 2:39
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    @romkyns: Only in comments. It's quite broken; they use different code for the comments, the live preview for posts and the actual post formatting on the page. Comments even come for free with escaping that's nowhere else to be found :-)
    – Joey
    Commented Jan 12, 2011 at 15:59
  • B for Windows boost drive.
    – Bratch
    Commented Jan 17, 2011 at 2:45

Ah... the good ole days.

A: was the first disk device, B: the second, and so on - in CP/M. As somebody else posted, this ran on the 8-bit 8080 and Z-80 machines that pre-dated MS-DOS.

MS-DOS in turn was a 16-bit (8086) knock-off clone of CP/M, or more specifically, CP/M-86, so it used the same drive letter conventions.

Back in those days the only conventions that were pretty much universal were set by CP/M (for example, the disk naming on the Apple-2 and Tandy TRS-80 was something different again... I once used a TRS-80 with 4 floppy drives... oh the power!).

With something like CP/M the first hard drives, when they came out, just appeared as the next available drive letter.

MS-DOS allocated 2 floppy drives pretty much universally and placed the hard drive at C:. Either you had two floppy drives, or the system "virtualised" access to a single drive - an application could access A: or B:, and the system would ask you to swap the physical floppies in the solo drive as necessary. This both increased software compatibility for cheaper systems (floppy drives were not exactly cheap, and software generally used a separate disk for the software, and another for your user data) and made e.g. copying floppies a lot more convenient (still annoying, mind...). This was still back in the day when hard drives were ridiculously expensive (a 6 MiB hard drive would cost around $5k of then-dollars, and a floppy drive around $1k) - most PCs didn't have one.

This convention has been preserved since, though as pointed out here numerous times - in windows - you can change this. As far as I know in MS-DOS you can't.

Slightly off topic:

Back in the 1980's it was pretty common on these early machines for disks to be a mixed bag of different types with legend incompatibilities. The IBM-PC imposed a bunch of standards with the format of its 5.25 inch floppies - before that you had all sorts of different arrangements with hard and soft sectoring, number of sectors / track, number of tracks, etc etc.

Data interchange between machines using floppy disks was a very hit and miss affair, which only got sorted with MS-DOS and the IBM-PC. The most reliable way to transfer files between machines back then was to write some programs (in assembler) to to file transfers using the serial port, and then wire them up. It was never very slow, because files were never very big.

My favourites from then were the high density drive my employer bought - it was an 8 inch floppy attached by a huge long ribbon cable. I think it stored about 200K, which was huge when developing software on a machine with 2 floppy drives, each of them 89K. Compiler on one, source and object code on the other. A compilation took minutes while the drives clanked away.

At the time the IBM PC was introduced you could get it running either CP/M-86 or MS-DOS, and there was no clear indication that one or the other would win. Eventually MS-DOS won the day - some bundling deals might have had a bit to do with that. The first IBM machine my employer bought did make things a lot easier... and the port of stuff to MS-DOS was pretty easy - mainly because of the same conventions for disk drive letters, and also because the system calls to read / write files and populate file control blocks (remember FCBs and the weird format they had in memory?) was the same.

C: wasn't rigid in MS-DOS as the first hard drive. Heath/Zenith MS-DOS had AB reserved for 5" floppies and CD reserved for 8"floppies.

The first default hard drive partition was E, Whether you had floppies installed or not. Their version also allowed 16 partitions on a single hard drive.

I think they may have been the first multi-booting systems as well. Hot-keys during the boot process would allow you to boot from any drive or partition, allowing custom environments and launching CP/M-85, CP/M-86, MS-DOS, Concurrent Dos or HDOS.

  • 11
    Remember the odd disk formats? You could squeak 420K onto a "360K" floppy disk by playing with the number of tracks and sectors. 800k on "720k" floppy disks, etc. Commented Jan 12, 2011 at 16:24
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    Some drives would have enough mechanical clearance internally that with a s/w tweak you could get about 2 extra tracks on and boost capacity by about 10%. You had to buy the exact right brand of drive, though. Commented Jan 12, 2011 at 22:39
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    I never ran across any 5.25" drive that wouldn't let you do 82 tracks on an 80 track disk. I suspect that it may have been a problem on early "compatible" (rather than clone) models though... Commented Jan 13, 2011 at 19:53
  • It was a reimplementation of CP/M-80 as the whole reason for doing that was that CP/M-86 was not available yet. Commented Dec 25, 2023 at 23:38

A and B were used for floppy disk drives. As far as I'm aware, there are two because the older computers (with no hard drives) would boot the OS from A:\ and run applications from B:\

  • 13
    Actually, there are two of them because the [much] older computers only had one floppy drive and the B: drive was used as a "virtual drive" to allow copying a file from one disk to another in a single-drive system. Commented Jan 10, 2011 at 9:47
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    It was used for both. If you had two drives, they would be assigned A: and B:. Also, after booting DOS on a floppy, you could usually take out the disk and put in DisplayWrite or whatever, and a lot of those programs had second disks, so you didn't have to switch them all the time. But yes, if you didn't dumpster-dive for a second floppy drive, you could use it as that virtual drive, and it would read like 1K at a time (seemingly) to copy data from one disk to the other.
    – Brad
    Commented Jan 10, 2011 at 17:01
  • 3
    Actually, I believe the most important use for two disk drives was copying floppies. I had an Amiga with one drive, and to copy a (880k) floppy, you had to switch back and forth several times, because the system had only 512k total RAM.
    – sleske
    Commented Jan 11, 2011 at 23:05
  • 2
    512k. Geez. You should have tried copying floppies on a machine with 64K of RAM and one floppy drive. Now that WAS an exercise in both patience and frustration. Commented Jan 12, 2011 at 7:47
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    sleske: Surely you didn't...ahem...copy that floppy... Commented Jan 12, 2011 at 14:00

They are floppy drives. If you're curious to the order of the drive assignments, wikipedia has more information http://en.wikipedia.org/wiki/Drive_letter_assignment

  • Back in the old days, a machine usually had a floppy disk drive and no internal hard disk. Your OS, e.g. DOS (Disk Operating System) came on a floppy which is why floppies are A and B and come before the hard disk, C. Commented Jan 12, 2011 at 21:11

A: is the 5.25" 1.2Mb removable magnetic media, and B: is a more modern 3.5" 1.44Mb drive in my coding machine.


If I remember correctly on my DOS systems they were used for floppy drives but B:\ was not generally not an actual physical floppy disk but rather a virtual one.

It was a virtual floppy used when copying a floppy when there was only one drive. So you would copy from A:\ to B:\ but keep taking the source and destination disks out of the same drive as the memory buffer got full.

  • 11
    Poor people only had B:\ as a virtual. People with money had two floppy drives and the letters to use them.
    – random
    Commented Jan 11, 2011 at 22:51
  • @random Later on, people had A: and C: and you only really needed a B: if you wanted both sizes of floppy. Commented Jan 12, 2011 at 0:00
  • A and B were floppy drives. They could be different sizes but need not be. (eg 8 inch and 5.25 inch). Commented Jan 12, 2011 at 7:44
  • 2
    I developed the text adventure "Shades of Grey" on a machine without a HDD, but with dual floppy... OS, Editor and AGT parser on one drive, source code on the second... and the PC had a built in 1200/75 modem for "cloud storage" (e.g. uploading my files to Compuserve)
    – Mark Baker
    Commented Jan 12, 2011 at 9:59
  • @Mark Baker HUGE +1 for mentioning "cloud storage" via Compuserve from back-in-the-dawy :) Commented Jan 15, 2011 at 7:42

While it is clear that these are used for legacy floppy disk drives, I should note that they're still used to this day by default for these ancient storage media:

Floppy drive connected to laptop Drive A: in Windows 7


A: was the boot device in the 8-bit CP/M operating system. It predates even the earliest MS-DOS. If you had a second drive, it was B:. A third would be C:, etc. Systems didn't have hard drives then. CP/M was originally targeted to the 8080 processor. CP/M was created in the 1970s. The very first small winchester disk drive was the ST-506 which boasted a generous 5 MB (five megabytes) of storage. They were expensive but they were fast (compared to what we were used to). Oh, and the primitive file system didn't have folders or subdirectories. There was no A:\. It was just A:.

  • 1
    AFAIK CP/M was more popular on the Z80.
    – bart
    Commented Jan 12, 2011 at 22:58
  • 1
    CP/M may not have had directories, but it did have 16 "user areas", so you had A0: though A15: as separate namespaces for files.
    – camh
    Commented Oct 20, 2013 at 7:36
  • According to pcmag.com/encyclopedia/term/st506 the transfer speed of the first harddisk was 625 Kb/s. If I recall correctly the transfer speed of 3.5" floppies were around 30 Kb/s. So at least a factor 20 speed difference Commented Dec 25, 2023 at 23:43

I feel old and I'm only 26. A and B traditionally are floppy drives. C traditionally is the main disk and D traditionally the CD-ROM

My computer at home has CDEF as hard drives and G as DVD RW

  • No, D was not the CDROM. D: existed way before CDs were commonplace.
    – Jé Queue
    Commented Jan 11, 2011 at 1:45
  • 3
    D only became the letter for the CD-ROM in systems that had just 1 HDD and didn't specifically assign a letter to CD-ROM. D for the CD-ROM was common, but not traditional IMO. In fact having D as the CD-ROM could be a problem/pain if you later installed a 2nd HDD which caused drive letters to change... Windows (and possibly other software) would record where they were installed from and request the same drive letter - but it had now changed.
    – MrWhite
    Commented Jan 11, 2011 at 23:41

On my first PC in 1992, A: was the 3.5" floppy, and B: was the 5.25" old-style floppy. Other machines had two 3.5" drives, and having two floppies was fairly common, hence hard drives starting at C: to leave room.


They are a legacy from the DOS days - A: and B: were the floppy disk drives that were standard on the original IBM PC. C:, the hard drive, was the third disk. Today, of course, nobody has floppies.

  • Hey! I have floppies! And I'm not "nobody". But nobody who doesn't deal with legacy/antique equipment has floppies... Commented Jan 10, 2011 at 16:04
  • 1
    @thrillscience - I felt vaguely guilty even while I was typing that. At my last job one of the hardware engineers had a 3.5" floppy USB drive so he could run one of the old schematic capture programs in DOS mode on a PC. Or so he claimed, I never once saw that drive move from under its thick coating of dust.
    – mtrw
    Commented Jan 10, 2011 at 16:28
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    I need it for some old MIDI drum-machines and sequencers that have floppy drives, but otherwise still work well. Commented Jan 10, 2011 at 18:21
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    @RBerteig - the more astounding part of that is that you have a working PDP-11.
    – mtrw
    Commented Jan 11, 2011 at 9:20
  • 1
    IIRC Robert X Cringely said in Triumph of the Nerds (whichever one was the documentary of Accidental Empires, which I have somewhere) that the drive letters came from CP/M, which pre-dated DOS. Commented Jan 11, 2011 at 23:00

A:\ and B:\ still defaults to floppy drives, of course only visible if you have one installed (only have that for fun in some of my virtual machines)


Lots of good answers already, but they all refer to floppy drives in the past tense. The fact is that floppy drives are still sold, though I have no idea who buys them.

Another fun fact: if you have a single physical floppy drive installed, you actually have both an A: and B: drive. If you do

 copy a:bigfile b:

the system prompts you to insert the A: or B: floppy as needed.

Primitive by today's standards, but still a step up from storing data on audio cassette!

  • 3
    There is a huge number of industrial machines that use 3.5" floppy as a means of getting the code to run. Usually CNC's and a lot of robots.
    – McKracken
    Commented Oct 19, 2013 at 19:13

A:\ and B:\ were used for the floppy disk drives (remember them?) ;)


The first pc I used professionally was an Apricot Xen. The internal hard drive was A: and the 3.5" floppy was B:. This was back in the day of MS-DOS 2.11, before Ibm pcs came along.

  • 5
    It seems unlikely you were using a MS-DOS based computer before the advent of the IBM PC. The Model 5150 was introduced in 1981 and came with the Microsoft developed PC-DOS 1.0.
    – tadman
    Commented Jan 12, 2011 at 5:36
  • 3
    It looks like MS-DOS 2.11 didn't arrive until March 1984, so IBM PCs must have been around at the time. Memory fading fast... :-)
    – dr-jan
    Commented Jan 12, 2011 at 14:09
  • 1
    In Japan there was also the NEC PC-98, which always uses A: for the boot drive. So if you booted from the hard drive, drive C: would be used for the floppy.
    – Yuhong Bao
    Commented Jan 12, 2011 at 18:25
  • en.wikipedia.org/wiki/Apricot_Computers 1985, not IBM PC compatible, ran MS Windows 1.0. Commented Jan 13, 2011 at 20:03
  • The idea of a drive letter followed by a colon predates DOS, it was used in CP/M also.
    – Warren P
    Commented Jan 3, 2013 at 14:31

Further to what the drive letters A: and B: were used for originally, here is how I use them now.

You can use the DOS command "subst" to assign an unused drive letter to a path on some other drive.

For example, I have multiple hard drives and directory structures that go many levels deep.

I use A: for my current project data folder, and B: for my current project code.

subst a: "d:\my documents\_PROJECTS\2024\SomeCustName\switches"
subst b: "f:\coding\_PROJECTS\2024\SomeCustName\bin"

In order to automatically run those subst commands whenever I boot my computer, the two lines are included in a text file, called autoexec.bat, that is stored here: C:\ProgramData\Microsoft\Windows\Start Menu\Programs\StartUp\autoexec.bat

The two drives appear in Explorer as the A: and B: drives, but they are shortcuts to my current project folders.

When I change projects, I update those subst commands.

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