I've never seen single sided 3.5" disks before. Were they specific to a particular computer?

Never seen a single sided 3,5" disk?

For 5 1/4" dd these were available. However, the 2nd side was also usable in many cases. You could turn it around and write it in a single-side floppy (after punching out the write-protect hole on the right position).

You likely will need to cover the drilled holes on the DD disks, as some drives may refuse to format them at 720K even with the /f:720 parameter if they have reason to think the disk isn't a DD disk. More than likely, you'll get the infamous "Invalid media or Track 0 bad" if you try to format them without covering the hole. Or, even if you do manage to get them formatted without covering the hole, the drive might refuse to properly read them afterwards and say the disk isn't formatted properly. The only exception to the above is if you use them exclusively in DD-only drives, in which case it's a moot point since they don't have the switch detecting the second drilled hole anyway and thus won't even care if it's there or not.

Something else that MIGHT be a good idea (but perhaps not absolutely necessary if I am mis-remembering my facts) is to demagnetize the improperly-formatted disks first using a strong neodymium magnet. Pass it over the disk radially outward from the center to the edges, like the spokes on a bike wheel. Do this until you have gone over the entire disk. Then, cover the hole and try formatting it. IIRC, 1.44MB disks use a stronger magnetic field than 720K (Or it could be the other way around, I admit it's been a long time since I last heard that), thus just reformatting the disks as 720K without demagnetizing them first may cause any remnant 1.44MB magnetism to eventually "bleed back through" and cause data corruption. Demagnetizing the disk using my stated method should wipe out any of this remnant HD magnetism and allow the disk to work normally again at DD specs. (Aside: This same method also works before using a 5.25in HD drive to reformat a DD disk, a task that is normally notoriously unreliable)

As for the single-sided disks, the first few models of Mac used 400K single-sided 3.5in disks. IIRC it wasn't until the Mac Plus or Mac II that 800K/Double Sided support came along. There were a few other (Non-IBM) PCs that also used single-sided 3.5in disks, but I can't recall models off the top of my head. I know the old DOS format program does have a parameter to format only one side, and on my Toshiba T3100 from 1986, it tries to format every disk as single-sided by default unless I specifically use the /f:720 parameter (It does have a double-sided drive, I'm thinking age has made it a little senile though and it's forgotten that it's double-sided. 🤣).

So I have no idea where or how the previous owner got single sided disks and for what purpose, unfortunately. I was a bit surprised to see quite the variety of disks, but since all but two were free of bad sectors, I'm not complaining.

I did format the 720 k disks as 720 without any problem; DOS and my internal drive didn't really care that much about the extra disk hole. However, I tried disks on my only other floppy drive: a Y-E Data USB drive (one of the few USB ones that supports 720 kb disks) and it was confused by the disk until I covered up the hole. So that's probably the best bet overall.

I don't have any real 720 k drives around, so I won't be able to test data/format integrity between the two drive types. If this situation changes, I can look into the "wipe with a magnet" method and reformat after.

tirondkflt, I did find the single-sided parameter for format, as mentioned in my original post. The exact error message I received was "Parameters not supported by drive." I've tossed some data onto one of the single sided disks (formatted as double-sided); It read back fine after unmounting and re-inserting the disk.

On some drives, if you use the 720k switch but the hole is uncovered the floppy will be formatted at 9 sectors per track on high density. You won't know until you try the disk on a DD drive or a computer that does not support HD.

Errius wrote:

I've never seen single sided 3.5" disks before. Were they specific to a particular computer?

They could be used on PC hardware as well I guess? I have a couple SSDD disks (which are obviously very old) and these were already hard to find in the wild over 10 years ago. I didn't even know these existed till I ran into a couple and tried to find out why these existed. Perhaps they were used for some Atari or something or they were simply sold as such as these disks may have been "downbinned" regular 720k floppies due to the magnetic disk being deemed not good enough to hold data on both sides.

Iirc mine were Sony 3.5in SSDD disks (or at least one of them).

I would however cover up the artificial hole in the reformatted 720K disks. Just use some masking tape 😀

The only major consumer-level use of single-sided 3.5" diskettes were by Apple (in the original Macintosh) and early models of Atari 520ST.

I've got a big box of formatted, blank (empty) single-sided double density 3.5" floppy disks in our garage. From what Dad said a few years ago, they're not Atari disks (though he did work at Metacomco in the early Atari ST days) but came from something later, either PC related or some 68k based system (probably a UNIX type one).

All blank, unlabeled disks packed in rows in a brown cardboard box with a cardboard spacer in the middle. (they look unused to me, just slightly loose and disorganized in the box from some being taken out previously)

They also seem to format OK to 720k DS disks, but I haven't torture tested them. It will not format them to 1.44MB DSHD disks, throwing error messages when I attempted it.
(given single-sided 5.25" disks often work as flippies or double-sided disks, I doubt there's much/any difference with SS disks other than formatting and maybe quality control: it was the single-sided drive mechanism that was significant: being cheaper, available earlier, and potentially more reliable or easier to harden for critical reliability: like specialized mainframe or military applications)

I know Atari started off with single-sided drives simply for the cost saving measure. (and as they were competing with Apple, who was using 400kB single-sided 3.5" disk formats for the Macintosh)

Windows 98 seems to recognize them as 720k DSDD disks with 366 kB free, but the format used can't be read or written to by DOS/windows. (either due to only one side being formatted or due to it being an incompatible partition format ... maybe DOS or CP/M like but big endian format data, like Atari TOS does: I'm not sure how typical 68k UNIX disks are formatted)

They're stored right next to a box full of Mizar/Integrated Solutions documentation with manual for a DEC 68k20 VME bus system. (and I remember dad had a big rack mount server/minicomputer like system set up around 1993 in the garage, I think it was surplus hardware he'd had in storage from several years prior, and he'd left Mizar around 1990, I believe so it probably got surplussed then)

So that style of disk might have been more common in specialized market sectors in the mid 1980s. I'll need to check the date on that manual in the garage, but I believe it's similar to this system:
http://bitsavers.informatik.uni-stuttgart.de/ … inary_Jan86.pdf

Edit: some disks do seem to have MS DOS readable data/formatting on them, and 720k DSDD formatted (or a format close enough to MS DOS files to read, even if the files might be interpreted incorrectly). I did notice some of the disks had write protection slots switched open but didn't think much of that initially, and most do appear to be unreadable or unformatted. (some may be single-side formatted and unable to be read properly) So maybe they were a mix of unused and discarded/surplus disks.

I'll have to set the ones with intact data aside and see if they are actually Atari TOS format disks or not. (or maybe Xenix formatted disks, though I think all of Dad's own 68k based Xenix systems were 8" and 5.25" based ones, SCSI drives aside ... maybe he ran the x86 version of Xenis at some point, too)

And I know the TOS (68k GEMDOS) has a very similar filesystem and disk format to DR-DOS and MS/PC-DOS, though not directly compatible. (close enough to make translation software relatively simple on the ST) Though I've never seen what happens if you try to read a TOS disk as if it were MSDOS formatted.

Single-sided 3.5" disks are actually not that rare. Over the last few years, I often bought boxes of DD disks used for Amiga games and about 5% of these disks were single-sided disks, some even Commodore-branded. 😉 They all format as double-sided just fine, as to my knowledge, there were no real single-sided disks. They all have the magnetic coating on both sides. I assume they simply sold disks as single-sided if one side failed quality assurance.

I do want to clarify that I have hardly ever bought 3.5in floppy disks because it was easier (and cheaper 😜 ) to get them donated to me for free 😜
There were just a couple of floppy disks amongst them that were SSDD.
With finding them in the wild, perhaps I should have clarified it more clearly. I meant more like what was actually on the market (or even to be found locally) vs stuff that was never sold and stuck in a wharehouse.
The 2.88MB disks were somewhat easier to find, even though I only ever seen a seller sell them locally on 2 occasions (2nd time I didn't get any as it were the exact same disks as I had already bought and I had plenty of those already).

Yes, the SS disks indeed had a magnetic layer on both sides. My guess is that it is kinda similar to how CPUs are sometimes binned lower on purpose because of relatively good yields.

Kool kitty, cheers for the extra background information 😀
Back then I was fascinated by all of these different media. These days we basically have only USB sticks and flash cards, but back then we had floppy disk, zip disk, super disk, the sony disk of which I have temporarily forgotten the name, floptical, the many different kinds of cartridges (SeaQuest and the weird IBM cartridges I have laying around (iirc it was a WORM disk something)).
It was really dynamic back then 😜