It is my understanding per the explanation of the guy at the Apple Store that when you disconnect your Time Machine drive your computer's hard drive is used to temporarily store backups until that drive is reconnected. The length of time you could go on with this being effective is directly related to the amount of free space you have on your internal drive. In my case, being that my MacBook Pro is not always at my desk, the time between backups could be up to 12 hours (excluding being on vacation). My plan is to take a bunch of the Apple Loops for Soundtrack Pro and video source files and particle systems for Final Cut Studio and dump them onto a 160 GB external bus powered drive (OWC Mercury On The Go from macsales.com). I also use that same external drive to capture video onto and store my render files. As my Time Machine drive I'll use a 750 GB Mercury drive connected via FireWire 800 each night. I would also like to find a way to backup my external 160 GB. Ideally it would work like Time Machine. But is Time Machine only capable of backing up what's on the primary OS drive?