Unidisk 3.5 won't write sometimes

["Stephen Hartz" <SHartz@columbus.rr.com>]

I have a very dependable 3.5" Unidisk drive on my trusty GS that has
peerformed flawlessly for 15 years.  Sometimes recently, however, why I try
to save a file to a diskette, I get the message the disk is full.  But it is
NOT full.  Subsequent attempts at saving also fail.  If i try later, it MAY
work...
Any ideas?? Drive speed?  (How do i adjust)  tried other diskettes- no
better, BTW.

Thanks!

Steve