hehe it's easy. just take the bolt spindle thingy out and replace it with a clock. you can buy the motor/arms/battery compartment as a kit at almost any hobby place (Michaels, Hobby Lobby, etc). ive never seen him do it with multi-platter drives, just old ones (5+ years). like the last one was a conner 120mg single platter drive.
you have to secure the platter to the pcb, the motor cant turn the platter itself. you just poke the motor through to the hole (motor is on the bottom of the drive) and attach the arms. may require some glueing for it all to stay in place.
you can put numbers on the platter if you want (or roman numerals, why am i explaining this part? you know what a clock looks like). makes neat conversation piece.