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I'm having this debate with a friend who is looking for a box of single-sided, single-density 5.25 floppy disks on eBay for one of his old computers.

I told him he could buy any 5.25 floppy disks he wanted, since both properties (sidedness and density) were a property of the disk drive, not the floppy. Floppy disk manufacturers certified both sides of these 5.25 disks, regardless if they were sold as SingleSided or DoubleSided. This was because there wasn’t any standard on where (top or bottom) the read/write head is in any particular drive. Density, likewise, depends on the drive – it’s just a different logical structure to how the tracks are laid out on the disk during formatting. Physically, there’s no difference between a SingleDensity and DoubleDensity disk.

He says I’m wrong about this. He thinks there’s a difference, particularly on the physical magnetic medium of the disk itself (a “single-density” disk isn’t capable of being formatted in the “double-density structure”.

So who wins this argument?

(I should clarify, we're not talking about HD disks, we both agree there's a physical difference with these).

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“Floppy disk manufacturers certified both sides of these 5.25 disks … because there wasn’t any standard on where (top or bottom) the read/write head is in any particular drive.” I don’t understand how this can be true. If some drives accessed the top surface of the diskette and some accessed the bottom, then disks written on one type of drive wouldn’t be readable on the other, right? What am I missing? – Scott May 2 '13 at 17:38
I actually looked that part up on Wikipedia. “Single sided disks were coated on both sides, despite the availability of higher-cost double sided disks. The reason usually given was that double sided disks were certified error-free on both sides of the media, but architectural differences among computer platforms negated this claim, with RadioShack TRS-80 Model I computers using one side and the Apple II machines the other.” I actually looked that part up on Wikipedia. – dvanaria May 2 '13 at 18:27
@Scott: Nothing. You're absolutely right. Disks written on one type of drive wouldn't be readable on the other. There was no standard and so there were multiple competing formats. Back in the day, I'd buy whatever was cheapest and had a special "nibbler" for adding write protect tabs to single-sided disks so I could write on both sides. – David Schwartz May 2 '13 at 18:29
It depends somewhat on the specifics of the drive, as there were a number of different 5.25" SD drives produced, with different characteristics. I wouldn't be surprised if some of the earlier ones would not work with "modern" DS/DD media, but I would expect that most would. – Daniel R Hicks May 2 '13 at 23:57
Your friend may be concerned about the fact that all available DS/DD media are probably "pre-formatted", but that's no biggie -- you can easily reformat. (However, I do recall that a few early drives/systems could not format their own media -- that would be a problem.) – Daniel R Hicks May 2 '13 at 23:59

Density is definitely different, single-sided or double sided floppies (mostly) aren't. Formatting SD disks as DD was a sure way to lose data quick.

If you have a single-sided drive with a double sided disk you could literally take out the floppy and flip it around and use the other side to store your data. All you had to do is cut a piece in the side of the disk to make sure your disk wasn't write protected.

I haven't seen single-sided floppies, but if they existed they probably used the head on the top side since all single-sided drives I've seen had their head on the top side.

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I don't think there was any physical difference between SD and DD disks, since they both used the same iron-oxide coating. The only difference was the label on the box. – dvanaria May 2 '13 at 18:31
Single-sided PC drives had the head on the bottom. And the "single-sided" disks looked essentially identical on both sides because an access slot was required so that the pressure pad could offset the pressure of the read/write head. – Daniel R Hicks May 2 '13 at 23:46

There's another property you haven't taken into account: Hard sectoring vs. soft sectoring. Hard-sectored floppies have some formatting information built into the disk itself. Hard-sectored ones are seldom found, and only some of the early drives used them, but getting the wrong one will make it useless for your drive.

The difference between single-density and double-density floppies is manufacturer certification. You can often use single-density ones as double-density, but there's a greater risk of failure.

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First, regardless of the answer, if your friend is using an SD drive, disks labeled DD would probably work -- but not the other way around -- in theory.

As to the question, reviewing known logical disk formats, the two 5¼-inch formats that used both Single and Double density (Acorn and Atari) also both switched encoding schemes -- a function of the drive -- from FM to MFM; the latter has double the data density as the former on the same physical medium. So you are right.

Also, a quick search found these 8-inch disks which are labeled "Single or Double Density". That's not definitive, but adds weight to the argument.

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There are definitely single sided 5 1/4" disks. I remember waaaaay back when when I had a floppy go bad on my Apple ][e, I would always cut them open to see the magnetic media. Single sided disks had a coating on the side that is written to and no coating on the other side.

When it came to 3.5" floppies, when high density was established, it was cheaper for manufacturers to make all the floppies HD and just mark them as DD. I remember taking stacks of my old DD drives, line them up and drill a hole in the corner to allow them to be recognized as HD. Every once in a while, I would come across a DD disk that would not work as HD though. However, it was probably manufactured long before HD was common.

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