Moving External Boot Drive into Mac mini (Mid 2007) Some time ago the hard disk inside of my family's Mac mini (Mid 2007) failed. We purchased an external disk (Some iteration of the Western Digital 'My Passport'), installed OS X onto it, and have been using it as the boot volume through USB. The computer has been running fine, albeit very slowly, though the computer now needs to be used more intensively and the bottleneck of a USB boot drive has made it impossible to do so.

So I ask you this: Will I need to format the disk once it's in the mini and reinstall OS X? Or will I just be able to place the My Passport's drive into the mini, select the now-internal drive as the boot volume and be alright?
Assuming the drive inside the My Passport has a SATA connection, then you can just take the drive as-is without any need for re-formatting or changing any data.

However you should look into the specific model of drive you've got — some external drives are manufactured with direct USB interfaces to save costs, rather than a SATA interface with a USB bridge.