Or.. with mobile drives, its possible it had been dropped/jarred, and it WILL knock the heads out of alignment. 

Easiest way to determine this is format a diskette in that particular drive. it should be able to read/use it. but no other system will. and vice versa. 

You can realign the heads by loosing the locking screw to the stepper motor and rotate the motor a little bit at a time until it locks in.