Im currently using a 3TB Airport Time Capusle to back up my internal HD. This is getting to its limit so I have bought 2 X WD Book Thunderport external HD's as I want to store my movies, pictures and music on WD Book No1 and use WD Book No2 to backup WD Book No1. The remainder of my files will stay on my Mac HD and be backed up by the Time Capsule. How do I configure my Mac to tell WD No2 to backup WD No1 only? Do I need specialist software to do this or does the Mac have something already to handle this? Thanks for your help.