This is very hard to figure out without having access to another such drive - and a clean floppy to test.

Since you seem to be getting issues with reading the floppy past the first few cylinders - which are less densly packed and easier to read - the drive can have dirty heads, this will prevent it from reading older, weaker floppies. If you have a 5.25" floppy that is empty or never used, try formatting it in this drive. Then do a write and read back test - if this works at least you know the drive is not faulty on electrical level, but it still doesn't mean the heads aren't dirty. Strong, fresh flux signals are easier to pick up.

Also this could be the heads getting stuck somewhere due to dirt accumulation on the sled/rails, that obviously also needs a proper clean. Could also be some sort of misalignment but I've never seen floppy drives going out of alignment on their own. BTW if you attempt cleaning do not remove or unscrew anything unless you know what you are doing - or you might misalign the heads yourself.

Thanks for the tips. Don't know if the reading heads are easily accessible. I'll take a look and if they are, I'll try cleaning them. Will also try formatting a floppy and see how write and read action is on that one.

OK, little update: I cleaned the reading heads with cleaning solution and I have some progress:

- Where I could not completely read any disk before (only the index, but always got errors when making an image), I can now read 360 kB disks without a lot of problems (a few are damaged, apparently, but that was to be expected). Also these that were not readable before. I could make images of almost all of them!
- The 1.2 MB disks however, are not readable at all. They are not even recognized (floppy drive tells me there is no floppy inserted).
- I did specify that the drive is a 1.2 MB 5.25 inch floppy drive in the BIOS (and it actually is such a drive, because it came from my very first PC, where I did read 1.2 MB floppies with it: the same floppy I still have and now want an image of).

Is it possible that one of the heads is broken, and as such the drive is "acting" like a 360 kb drive and not a 1.2 MB one?

TomVDJ wrote on 2023-05-12, 09:16:

OK, little update: I cleaned the reading heads with cleaning solution and I have some progress: […]
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OK, little update: I cleaned the reading heads with cleaning solution and I have some progress:

- Where I could not completely read any disk before (only the index, but always got errors when making an image), I can now read 360 kB disks without a lot of problems (a few are damaged, apparently, but that was to be expected). Also these that were not readable before. I could make images of almost all of them!
- The 1.2 MB disks however, are not readable at all. They are not even recognized (floppy drive tells me there is no floppy inserted).
- I did specify that the drive is a 1.2 MB 5.25 inch floppy drive in the BIOS (and it actually is such a drive, because it came from my very first PC, where I did read 1.2 MB floppies with it: the same floppy I still have and now want an image of).

Is it possible that one of the heads is broken, and as such the drive is "acting" like a 360 kb drive and not a 1.2 MB one?

No, a broken head would allow only one side of the disk to be read.
not being able to read 1.2MB disks could be indicative of an alignment problem.
Have you tried the imagedisk software ? IMD http://dunfield.classiccmp.org/img42841/imd119.zip
I've got no experience with correcting these kinds of errors.

TomVDJ wrote on 2023-05-12, 09:16:

Is it possible that one of the heads is broken, and as such the drive is "acting" like a 360 kb drive and not a 1.2 MB one?

DD floppies have tracks twice as wide as HD ones. So if your drive somehow got misaligned it is possible that heads are not positioned correctly for HD, but it's good enough to read DD format.

On most modern drives it's really next to impossible for the drive to misalign itself on its own. The screws holding the stepper motor (and track 0 sensor) are pretty tight and painted over for good measure. However the chips in modern drives can actually microstep the head - but only for the initial recalibration to track 0 on power-on. A dirty track 0 sensor can cause a shift in the head position that would result in "soft-misalignment" that is not permanent but persists over each period of use (until next power cycle).

So try blowing come compressed air through/near the optical barrier for track 0 and see if that helps. Blow some air on the sled "wing" that breaks the barrier as well, in case it's dragging some dust ball behind it. Hopefully that will be enough to fix your problem (remember to power-cycle). If not then full sled/rails cleaning is recommended, and if that doesn't help either then full realignemt procedure with a known good HD floppy.