i know for sure after the 2005.5 update you can use any hard drive. before then, they wrote a custom firmware to the harddrives and the software calls the firmware ID from the drive. if it's anything but merit0001 (or whatever) then it doesn't run.
people pissed and moaned about it enough that merit changed the software to eliminate the call.
you can:
edit the firmware so the drive ID says merit001 instead of samsung HM120
or
buy an IDENTICAL maxtor drive and transfer the logic board over
or
upgrade (if need be) of post 2005.5 software and load your own drive.
if you already have post 2005.5 software just pop in a new drive and re-install the software off the factory disks.