My situation: Currently my boot drive has a total of 500GB (2x250 RAID 0) and my ext. Time Machine drive is a 750GB external. The available free space in my boot drive has dwindled down to 80GB so I need to upgrade ASAP and have already put an order in for a TB (2x500 RAID 0). Given that here are my questions: 1) Even with the TB drive installed the amount of data that needs to be backed up will be significantly less near term. My Time Machine drive has 200GB of free space right now. That should hold me for at least 6 months until I'm ready to upgrade my TM drive. But will TM allow me to use a physically smaller drive than the boot drive it's backing up? 2) When I install the new drive I'll just clone my previous drive. Will Time Machine recognize the data on the new drive is the same or will it think new drive and duplicate the drive again? In other words, how smart is TM? Once I clone my boot drive and get my Mac running again can I plug and play my existing TM drive or do I need to reinitialize it first?