5.25 floppies that read but don't write

[Marc Verdiell]

I have connected a 1.2M 5.25" floppy to my computer. After a bit of jumper
learning and setting, it's recognized and reads my old DD and HD floppies
fine. But for the life of me I cannot write to it. Not under DOS, Win98, or
WindowsXP. Which all read fine. 

But can't add a file. It goes through to the motions and makes the noise,
heads moves as if everything went fine, but if I take the disk out and put
it back in, the file is not there. 

I can't format either. Fails after a while on the above OS'es with different
error messages without the head moving past track 0, suggesting it can't
even read back the first track it's trying to format. Formatting from
ImageDisk or OmniDisk looks like it works (head goes through the motions
over the whole disk). But nothing seems to be written on the disk: neither
utility can read the formatting back. Both HD and DD. 

On the same cable there is a 3.5" A: drive which reads and writes fine.
Can't be the controller since it works on the 3.5" drive? The cable maybe? I
tried several. Anyone has had this happened ever? Time to bring out the
o'scope?