Since you want to use it for time machine and depending on the amount of data you want o back up, I would recommend a drive with Firewire 800 if you can swing it. Firewire is easier on the CPU and faster on the throughout. I like the GoFlex drives by Seagate, as the connectors can be changed to address the needs for multiple systems. Little known fact by most is that the GoFlex connectors can be connected to bare Sata drives which allows for easy cloning of internal prior to exchange without the need for a special external case.