Does anyone know of a successful way to back up a USB disk attached to an Airport Extreme? I am using one as a centralized iTunes Server, I know Time Machine will not support backing these up, but I can't seem to get Carbon Copy Cloner to do it either.
What are you trying to back it up to? Do you want the backups to happen automatically? Do you want the backup to happen regularly? How often? Do you want the solution to be free or are you willing to pay for program or service.
One simple solution is to disconnect from the Airport Extreme and plug directly into the Mac and clone the drive from one to another. You can also connect a second drive to the Airport Extreme with a USB hub and whenever you add content to the iTunes server, simply copy it to the other disk.
There is folder synchronization software available such as ChronoSync.
There are online storage solutions like DropBox, Mozy, JungleDisk (probably pretty expensive for lots of data) and online backup solutions like SugarSync.
You might want to look into CrashPlan. Crashplan offers online backup to their servers and also backup to your own extra storage. You might be able to set it up to backup from one network drive to another. The issue with Crashplan is that the backed up files are stored in a proprietary compressed form. You will need to recover files if your iTunes server disk ever dies.
You can also consider a hardware solution by using a RAID with mirrored disks. That would be automatic.