My 1tb LaCie external drive is about to fulfill its purpose. I'm going to transfer most files and network preferences over to a new 2012 iMac when I get it. Using Migration Assistant. I don't want the old system or apps from my current PPc G4. When that is safely accomplished, how do I clear this data from the Time Machine drive so I can start fresh? My current HD is a 120Gb and I think I have about 250-300Gb of backed up files on the LaCie.