I am looking at some OWC external drives to use with time machine on my new iMac. Is it better to go with a bus powered drive or one that has it's own power supply? I like the idea of only having one cable back there to keep with the clean look of the iMac but will go with an AC adapter if need be. Also I have a 1 TB hdd and only using a fraction of it right now. Do I need to go with a 1 TB ext hdd or can I save money with something smaller? Thanks.