Do you have a spare IDE/FDC controller card for testing ?
I once had similar problems (->read/write errors) in a Pentium 133 and an ISA floppy controller solved them.

Are you formatting them in a 80-track 1.2MB drive (narrow head) so a 40-track 360KB drive (wide head) has problems reading them? If so that's a known issue.

@Jo22
I use the onboard FDD controller of the DFI (nForce4) but I have some older stuff like S.478 with Pentium 4.
The floppy drive in the IBM itself is working perfectly.

@Jepael,
It could be, I'd go check what the model number is of the drive in my PC.
Sometimes I am able to do a directory listing on the IBM of that floppy but reading would give an error, either a "general failure" or "sector error".
I think that might be the issue since the drive will read both the IBM floppies and the floppies formatted on itself.

Yes, that's a 1.2 MB drive. 360 KB floppies formatted in a 1.2 MB drive will not read correctly in 360 KB drives.

You're going to have to get hold of a 360 KB drive for your new PC, or temporarily install one of the 5155 drives in it.

I have successfully formatted and written 360KB disks in 1.2MB drives, but my method is a bit tedious, especially if you have to regularly modify the contents of the disk. Here's how I do it:

1. Take the disk you want to format, and run a strong rare-earth magnet across the entire surface radially from the center outward in straight lines, like spokes on a wheel

2. Format the 360K disk in the 1.2MB drive, then use that same drive to write the desired files to the disk

This should make the disk readable in the 360K drive, and you should also have no problem adding or modifying files using the 360K drive.

HOWEVER, BE AWARE...if you want to use the 1.2MB drive to add more files to the disk or change files already on the disk, you will need to repeat the entire process...wipe with magnet, format, and write the new files to the disk. If you don't do this, then the changed data on the disk will be unreadable in the 360K drive.

Can you use the magnets from old hard drives to do this? I have a lot of those.

I think these should be fine too.
I have a big neodymium one (25kg strength) and I'll give it a go, sounds like an interesting idea.

+++

Hilarous, it actually works! 🤣 Wiped the floppy with a magnet, Windows then reported that the floppy had an 'unknown format', I formatted it with the format A: /4 command and that did the trick.
Great tip!

Errius wrote:

Can you use the magnets from old hard drives to do this? I have a lot of those.

That's what I typically use, but any rare-earth magnet should work, or even a plain ceramic magnet if it is a solid chunk (not a flat refrigerator magnet) and is suitably strong enough. I personally prefer and recommend rare-earth instead of ceramic simply because of how much stronger they are, making it a better "sure shot" at working. Also, magnets that are physically larger are better, since they cover more area and thus reduce the number of times you have to run it across the surface of the disk.