I'm having a strange problem with an old 5.25 floppy drive and would like some help troubleshooting, at this point I would be interested in finding the cause even if it's not easily fixable.

The drive is an ALPS DFC642B01B (5.25 HD/1.2M) from around 1987, probably from some IBM PC of that era. It appears to work fine, is really silent and reads correctly every floppy you throw at it. I successfully used it to image many of my original games last weekend. However, trying to format a disk always ends up the same: it goes smoothly until around 55%, at which point the head starts going back and forward as it fails to verify the tracks it's writing. This goes on until around 60% when formatting resumes without issues until the end of the disk. The result is a formatted floppy with a bunch of bad blocks, but these are known good disks that can be 100% correctly re-formatted on other drives.

Next, I tried writing a disk image with DSKIMAGE, and it succeeded without errors. When reading back the image, it struggles for a few tracks starting at around track 45 (which would be around 55% of the total 80 tracks), but after a couple of retries these sectors can be read correctly. Considering it can read original write-protected floppies without any issues, the problem seems to be that for some reason it's not able to write those specific tracks properly.

I first thought it may be an alignment issue, but as I mentioned it reads all my original floppies, as well as floppies formatted on other drives with no problems, and passes the alignment test on ImageDisk with flying colors for all of these floppies. So at this point I concluded that it has to be specifically a write problem.

My next thought was that the head may be dirty or broken, which prevents it to write correctly, but I made sure it's perfectly clean, and as I said it writes to all the other tracks just fine, this issue only occurs on a bunch of tracks starting at around track 45. If a head was bad, it would likely fail on every track or at least randomly. What is the difference when writing on track 45 vs track 3 or track 75, other than the head position? Maybe on that part of the disk it needs more "juice" and some electrical issue prevents that (such as dried caps)? Just pure speculation as I don't know much about floppy drive technology.

By the way, I tested with several motherboards/FDD controllers, cables, power supplies and floppies, all known good.