what speeds are they running at ? running two drives shouldnt slow down the computer .... just make sure u defragment enough
id run the 40 gig for the OS put a 10gig OS partition 30 gig for backup and the 80gig for music or switch the backup and music around
id run the 40 gig for the OS put a 10gig OS partition 30 gig for backup and the 80gig for music or switch the backup and music around