In Leopard on the Mac, every time you plug in an external disk you will get asked if you want to use the disk for Time Machine back ups. If you are constantly plugging disks and taking them out, this can get old after a while. If you want to skip these dialogs, you can. Open a terminal window and type in the following:

defaults write com.apple.TimeMachine DoNotOfferNewDisksForBackup -bool YES

Once you hit enter, Time Machine will no longer ask if you want to use new disks by default. If you want to change this behavior, you can repeat the above command and change the YES to a NO you will again be asked if you want to use a disk for Time Machine backups every time you plug in a disk.