Why is it that I see an A: drive and a C: drive but not a B: drive?

Is there a reason why the disk partitions start at C? And is it possible to change that letter designation?


Because back in the day of floppy disks, there were either two physical floppy drives (A: and B:), or just one physical floppy drive (A:) with one emulated (B:) so you could copy from disk to disk by exchanging disks every few hundred KB.

  • Can we change it ? – ukanth Sep 17 '09 at 10:15
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    The tradition, or make C: become B:? In Windows' Disk Management console, you can in fact assign hard drives the B: drive letter. But you can't reassign the drive letter of the system drive (usually C:) because the OS would go crazy if its own drive letter changed. – Andrew Arnott Sep 17 '09 at 10:17
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    Way back when, there was no such thing as an emulated floppy drive, and hard disks were expensive. Computers would normally come with two floppy drives. One of the real big complaints with the early Macintoshes was that they had only one floppy drive, and no hard drive. – David Thornley Sep 17 '09 at 14:22
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    And some older programs assume C: (a) exists and (b) contains Program Files. – Macha Jan 19 '10 at 8:02
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    @Macha Not older. Badly written. – kinokijuf Jan 27 '12 at 13:42

The A and B slots are very useful when you want to give a particular removable device the same drive letter each time it's inserted. Windows will never assign A or B to a device, but if you assign A or B to a device using Disk Manager, that drive letter will be assigned on future inserts.

I keep my source control database on a USB key so I can transfer it between multiple machines, and always assign it to B because I know that drive letter will be available on every machine. Finding this trick simplified my life greatly.

  • always available... unless your answer gets accepted! – Mikeage Sep 18 '09 at 6:58
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    -1, that's not why. – hasen Sep 18 '09 at 10:29
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    Wouldn't be available if you plugged it into my machine. I have 2 floppy drives... :-) – Brian Knoblauch Sep 18 '09 at 11:46
  • Exception noted. :) – Roger Sep 18 '09 at 18:16
  • I keep my Zip drive on B: (yes, I have a Zip drive ;-) – kinokijuf Jan 27 '12 at 13:39

Can we change it ?

Sort of. Some RAM disk drivers and USB tools allow to assign the long lost drive letter B:.

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    Some of us still actually have B: drives in our machines. I've got so many floppy drives laying around that I usually just go ahead and throw 2 in the boxes I put together. Surprisingly, it actually comes in handy from time to time! – Brian Knoblauch Sep 17 '09 at 12:15
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    What do you store on floppies that is still readable nowadays? – Anthony Giorgio Sep 17 '09 at 12:27
  • @Brian: i hate to break it to you, but you are an exception to the rule, and a rather rare one at that. :) – Molly7244 Sep 17 '09 at 14:05
  • I have a (working) PDP-11 in a back room that has a working 8" floppy drive and a 9-track tape drive. I also keep a few PCs around with working stale drives and OS installs. It does come in handy. – RBerteig Sep 18 '09 at 8:06
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    Still have to put drivers on floppies for OS installs sometimes (when hard drive driver required). Also use for moving small files around between machines that don't have free (or working) USB ports. Lots of little random stuff that us programmer/sysadmin people run into on a regular basis that a typical computer user may never encounter. – Brian Knoblauch Sep 18 '09 at 11:49

You can map a drive as B: if you want. Under computer management, go to storage/disk management. Right click the drive you want and choose "change drive letter/paths" You should be able to select B: as an option.


I use the B drive for mapping network drives when I've run out of other letters (surprisingly easy to do with USB hubs and such).

  • I really don't see why people map drives at all anymore. Full network paths can be added sort of like "favorites" to your network places. Then, you can just select those instead of having to remember exactly where a particular drive letter points to. I use tons of network drives, yet only have a single mapped drive setup (to support a legacy app that can't handle UNC paths). – Brian Knoblauch Sep 18 '09 at 11:50
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    If you're into the command-line, the drive letters are very quick to type. So, as long as your memory is good, you can save many keystrokes. – Chris W. Rea Jan 7 '10 at 2:39

I've seen Softgrid (now called Microsoft Application Virtualization (App-V) version 4.5) installs which use the B drive as it's hidden drive... It uses the drive to store the applications to run.

Here is how to do that: http://social.technet.microsoft.com/Forums/en-US/appvbeta/thread/d9d46885-d136-4ace-9cd9-3b881322b86a

jeah subst isn't a good idea but if you are on XP or older and need to change the drive letter to B: then go to HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices and change \DosDevices\Q: to \DosDevices\B:.


Disk Manager (part of Computer Manager) will allow you to assign 'B' to one of your drives.


My Windows XP box at the office has no C drive either. There was a bug in the text-mode setup version I installed from that was triggered by having a Zip drive (think 100 MB floppy) installed when running setup, resulting in Windows XP installed on drive E.

I've found it entertaining (and even occasionally handy) to have no C drive. Quite a few application installers have shown "quirks" when faced with no drive C at all.

  • And you can always use subst to create a virtual drive C if something truely requires it. subst c:\ E:\ – Jack B Nimble May 9 '11 at 20:03
  • @Jack, I kept a couple of network shares all set and ready to go to net use as C:. One was for a very stale product I was stuck maintaining that required some parts of itself be located in folders right at the root of C:. It was really convenient to not actually have a C: of my own for that project. These days I would just set up a virtual PC to develop and test inside, of course. – RBerteig May 9 '11 at 20:21

A: is for 3.5 floppy drives, B: was for 5.25 drives, noone uses 5.25 anymore so B: is no longer assigned.

  • Sorry, not correct. A: and B: can be any combination of BIOS-supported drives/sizes/capacities. – Linker3000 May 3 '11 at 19:22
  • of course it can, any letter can be any drive you'd like. but by default it's as i said. – acme64 May 13 '11 at 0:56
  • Have to disagree - there was no published standard that defined that as the default position. – Linker3000 May 13 '11 at 13:10
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    and yet, that's how a lot of system where setup – acme64 May 14 '11 at 2:01

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