To set up or adjust Time Machine:
iPods and iDisks can’t be used for Time Machine backups.
When Time Machine performs the first backup of your system, it may take a while. You may want to set up Time Machine in the evening so that the initial backup can be done overnight. Subsequent backups take less time because only items that have changed are backed up.
Note: Time Machine works best if you use your backup disk only for Time Machine backups. If you keep files on your backup disk, Time Machine won’t back up those files, and the space available for Time Machine backups will be reduced.