I use carbonite to back up my PC (Windows XP). We were running low on disk space on our home PC (down to 15 gig) so I went out and purchased an external hard drive. However, Carbonite will not back it up. I just want the external drive to be extra disk space. From their FAQ:

The current version of Carbonite backs up only the files that reside on permanent hard drives on your computer.

It will not back up network drives, external drives, and NAS (network accessed storage) drives. If there are files on a remote drive that you wish to include in your Carbonite backup, you should copy the files to a folder on your local hard drive. If the files are on a shared network drive, you could install Carbonite on the computer on which the network shared drive physically exists, and back the files up directly from that computer.

Check back soon for a Carbonite service plan that will allow you to back up your external drives.