Time Capsule: backing up external HD
I am using Time Capsule as my Time Machine to back up my iMac. It is also the Time Machine for my AirBook. I have an external Hard Drive, connected to the iMac by USB, on which I store my photos. How can I get the external HD to be backed up as well to the Time Capsule?
iMac, Mac OS X (10.7.4)