Some time ago I reviewed iBackup, though I cant seem to find the review on MacMod anymore. At any rate its a simple free utility that does exactly what you are asking. You schedual times and what you want to back up for each client and iBackup does its thing. It mounts the desired volume, performs the backup and unmounts when its finished.

http://www.grapefruit.ch/iBackup/index.html
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PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...