TM doesn't appear to make a local hourly image when your TM drive isn't attached. There goes all the love from laptop owners! I have two laptops, a MBP and a PB G4. The MBP was TM'ed to a 500Gb USB2 drive on friday night and has been connected since. This was done with existing data on the drive and without reformatting or any other messing around. I decided to get TM going on the PB, so disconnected the 500Gb from the MBP and connected to the PB. Time machine opened straight away, asked to backup to the 500Gb and proceeded to create a backup of everything on the PB. Meanwhile I kept the MBP on and running, mainly just web browsing. TM finished with the PB and on looking at TM, there was a (very short) history of the drive since it was backed up. It had also added a second subfolder within 'Backups.backupdb" which was called "<username> Powerbook G4", alongside the already created "<username> MacBook Pro". When I connected the 500Gb back to the MBP, gave TM a few minutes to think, and checked TM's history there was nothing new between "Now" and when the TM drive was last connected. I hope this answers some questions. Please Apple, address this for laptop users and provide some option to save images to a local temp location until the TM drive is reconnected.