Bit late but I'm just trying to figure out what you want:

Computer off: Printer off

Computer on: Printer on

Computer on but in sleep mode: Printer off.

The easiest thing to do is see if when the computer goes to sleep the PSU cuts power to all the drives or if it just tells them to spin down. If the former it's a simple relay solution but if it's the latter you need a bit more work running a transistor off the spindle motor power on a hard drive to open/close a higher voltage relay circuit.

...or poke around with a DMM and see what on the computer loses power when it goes to sleep.