I've been having a problem with Time Machine lately (OS 10.5.1). My computer is set up as follows: Internal iMac HD Firewire external HD (music) USB external HD (Time Machine) I have Time Machine back up the internal and firewire (music) drives. The initial backup takes up appx. 290GB of my 500GB TM drive. Subsequent backups function as normal, until a restart. After a restart, the next TM backup seems to want to do a full backup of my music drive again. This causes TM to erase all my previous backups on the TM drive and eventually give me an error that the backup is too large. I have been rebooting without unmounting either the TM or music drive. Should I be unmounting the music drive to avoid this issue?