TimeMachine with multiple disks (one at a time) I have backed up my Mac to an external disk using TimeMachine. When I plug in a different external disk the Mac still expects to backup to the first disk. Is it possible to backup to two external disks, one at a time?
You just have to select both disks as backup disks from Time Machine Pref pane, one at a time.
Edit:

Attach your first drive, open the Time Machine system preference, click the Select Disk button, and choose the attached drive as the backup destination. Initiate a backup and wait until it’s done.
Now attach the other drive, open the Time Machine system preference yet again, click Select Disk, and choose the new drive as the backup destination. Again, wait for the backup to complete. You now have two identical backups.

Taken from Macworld Article