Have you shaken the drive gently to see if anything is loose inside?  I ask this because there is a glass head guide that is glued in and literally falls off after some time.  I tried to fix mine but it cracked off and was chipped.  With a 5MB drive, you could actually open it as long as you didn't sneeze on it

Here's what the glass guide looks like and my feeble attempt to fix it.  If it doesn't go in perfectly, the head is misaligned and it will not work but it was worth a shot anyways.  If you hear something rattling around inside, you'll know what it is.