Head alignment is off on one of the drives. Either the USB drive or the one in your 486. When the alignment is wrong, you can read/write with the originating drive but not another.

kingcake wrote on 2024-03-29, 18:28:

Head alignment is off on one of the drives. Either the USB drive or the one in your 486. When the alignment is wrong, you can read/write with the originating drive but not another.

Is there any way to correct?

You'd have to manually realign the heads, if that's the issue. You can verify using IMD.

I can confirm that ist is possible to write floppys in Win11 and read the content in DOS. I use an external USB IBM drive under Win11.
So one of your drives is broken.
Try a third drive to rule out the problematic drive.

oLdStuffUser wrote on 2024-03-29, 21:54:

I can confirm that ist is possible to write floppys in Win11 and read the content in DOS. I use an external USB IBM drive under Win11.
So one of your drives is broken.
Try a third drive to rule out the problematic drive.

Of course it works. Fat12 hasn't changed since 1977.

fsinan wrote on 2024-03-29, 18:22:

What could be the reason? Is there a format difference in writing files on Win11?

In general anything newer than WinXP (except, maybe, 32-bit Win7) can't really format floppies properly. Read and write, yes, but not format. Why are you even trying to format the floppy on Win11?
Format the floppy on the DOS machine. Then, if you want to check, copy some files to it from DOS machine and try to read the floppy on Win11. If that works then just use the floppy as-is. If it doesn't then make sure both machines can read the floppy it wrote, after it was ejected and re-inserted. If that works, but the data is not readable on the other machine, you have one or both drives with misaligned heads.

USB Floppy drive can read the floppies formatted on systems floppy driver but it is not true for vice-versa.

USB floppy is not able to write in correct format. It can read its own disks. Not not compatible to 486 floppy drive. Is it a format issue or a hw problem?

So, a floppy formatted and written to on DOS machine is readable by that USB drive on Win11 - correct?
If that's the case, try adding some files to it on Win11 and see if DOS machine can read them. If not, or perhaps if the floppy now apprears completly unreadable in DOS, then I would still suspect head alignment. It's possible the difference in head postion is not so big so a drive with better (less noisy) electronics can be still picking up the flux (though probably with problems on the inner tracks) but the other one is just seeing garbage. But just becuase one drive looks to be working better doesn't indicate which one is misaligned. Could be both are, or perhaps both are just on the edge of the spec but on the opposite side of the track. You will need a known good drive and preferably some reference floppies to tell. For reference floppies you can use software that was distributed back in the day, it should've been written on well-aligned duplicating machines. Obviously the floppy must not have been written to since.

Format and write a floppy on your USB drive. Put it in your 486 and look at the track alignment with IMD. It's simple and easy to see if that is the problem. Read the IMD documentation.

Couldnt find any info to read metadata of a floppy disk.