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Is it possible to format a 3.5″ floppy disk in another size?

For example an original 1440 KiB floppy as a 720 KiB or 360 KiB disk thats (at least read-only) compatible with 720 KiB or 360 KiB drives?

As mentioned by the comments below, @Greenonline suggested that a 1440 KiB supported drive could possibly be used to format and write a floppy in a smaller format like 720 KiB or 360 KiB. The disk then could possibly be used as read-only in a drive that supports lower capacity disks only, such as 360 KiB or 720 KiB drives. Where the disk would have enough coercivity to hold the lower capacity data but an older drive would not have the capacity to write to such disk.

user3840170
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Bob Ortiz
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  • Most likely not possible, see https://retrocomputing.stackexchange.com/q/24565/4025 – Leo B. Jul 08 '22 at 20:10
  • Do you mean, put a 1.44 MB disk in a 720k (or 360k) floppy disk drive and use it accordingly? – Greenonline Jul 08 '22 at 20:32
  • @Greenonline I mean use a originally higher capacity disk formatted in a lower capacity format for specific older drives that only support that lower capacity. But I suppose not based on the mentioned answer about coercivity. – Bob Ortiz Jul 08 '22 at 20:36
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    I wonder whether a disk formatted for lower capacity in a 1.44 MB disk drive (which would have enough power to magnetise the 1.44 MB disk correctly) and then written to in the same 1.44 MB drive, could then be read (but not written to) without issue in a 720KB or 360 KB drive..? Surely reading wouldn't be an issue, just the writing to (because as stated in to the other question, the 720KB floppy drive wouldn't have sufficient power to write correctly to the 1.44 MB disk). If that makes sense... – Greenonline Jul 08 '22 at 20:42
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    If your question was about creating read-only disks that were written to in a 1.44 MB, for read only use only in a 720 KB drive, then it probably should be re-opened. – Greenonline Jul 08 '22 at 20:42
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    @Greenonline Interesting thought. Didn't think of it that way but not being able to write, makes sense. I will rephrase and reopen. – Bob Ortiz Jul 08 '22 at 20:59
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    Bob, Just one minor re-correction - It is MB for megabytes, not mB (which is millibytes). And there should be a space so "1.44 MB" not "1.44mB" (the same for the kB (which is a small k), so "720 kB", etc. – Greenonline Jul 08 '22 at 22:49
  • @BobOrtiz Please do not use a question for discussion. Also, it still is a duplicate, as there's no principal difference due media size – Raffzahn Jul 08 '22 at 23:42
  • No, an HD drive would write data with a bit rate too high for a DD drive to read. – Leo B. Jul 08 '22 at 23:59
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    I think this was dupe'd in error, given that the other question is about writing HD media in a DD drive, where the answer is "No". For this, the answer is "sometimes", as evidenced by people who put tape over the HD identifying hole, format the disk as 720K, and use it to transfer data from a newer machine to an older one. I can't remember if the "with varying success" part only applies to having the old machine try to write to the disk. – ssokolow Jul 09 '22 at 01:56
  • @LeoB.: An HD drive will write data at whatever speed it is delivered by the controller. If a controller is writing at the HD data rates, then the disk would be unlikely to be readable by any drive hooked up to a low-density controller, but if data were fed to the disk at the right speed, a low-density drive and controller would see flux transitions at the same speed as they would appear on a low-density disk. If different drives use different rotational speeds, it might not be possible for an HD controller to generate the right data rate, but a modern microcontroller certainly could. – supercat Jul 09 '22 at 17:05
  • @supercat That would require a chimeric system with a HD drive connected to a DD controller; are they electrically compatible? As far as I can see in the Wkipedia, the rotational speed of the 3.5" disks was 300 rpm. I believe that the implication of the question is "using standard means rather than homebrew electronics". – Leo B. Jul 10 '22 at 04:27
  • @LeoB.: I read the purpose of the question as being interested in the question of whether a high-density a disk would be capable of read-only operation with a computer and drive that were designed for low-dessity disks. The drives needed to write the disks are aleady not period-correct, but I think the question is whether, once disks were properly mastered, they could be used in period-correct fashion, and I would expect that not only would that be possible, but they could work better than standard disks. It was possible in the 1970s to write disks for the Apple with somewhat... – supercat Jul 10 '22 at 13:07
  • ...higher data density than standard, which would read back at somewhat higher data rates, if the data was written with a drive that was adjusted for a somewhat lower than standard speed, but the "smearing" of flux transitions by media would cause data to read back unreliably if things were pushed too far. Mastering high-density media to be read by low-density drives would seem like it could have been an interesting copy-protection technique. – supercat Jul 10 '22 at 13:13
  • @LeoB.: Adapting high-density drives for use with standard-density controllers, to format high-density disks to low-density format readable by low-density drives may require tweaking the rotational speed calibration and/or cable pinouts, but would not require anything particularly exotic. Using a microcontroller may allow more precise control over the formatting, and thus allow storing somewhat more data, than would be possible using a conventional drive controller, but it should be noted that "chimera" systems that combined a high-density controller with both kinds of drives would not... – supercat Jul 10 '22 at 17:11
  • ...have been terribly uncommon back in the day. If someone had a spare drive bay when upgrading a system to use a high-density drive controller and a high-density drive, it would have been common to leave an older drive installed because there would be no particular reason not to. – supercat Jul 10 '22 at 17:13

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