How to backup Airport Extreme USB drive with Time Machine I have a drive connected to the Airport Extreme using its USB cable. I want to back up this drive using Time Machine (or some other practical method, though I trust Time Machine more than other solutions.) 

Unlike most who have this kind of question, I do not want to backup my local machine to this drive.  I want to backup this drive, along with my machines local drives to my external drive using Time Machine.

There seems to be no good way to back up this drive, meaning all the data on the drive shared by the Airport is at risk if something happened to it.

In case anyone is unclear, I have 4 devices in this scenario, the Airport Extreme (AE), the USB drive connected to it (AE-USB), my local Macbook (MB) and the external drive connected to the MacBook (MBED).

I want to store files on AE-USB, and I want those files to be backed up somewhere, preferably using MB's Time Machine which backs up the MB drives to MBED.  I want to back up AE-USB onto MBED so that if something happens to it, I can restore it from the locally connected drive MBED.
Time Machine is only designed to backup internal and locally-attached drives. It cannot backup network-attached storage devices (such as the AE-USB). If you would like to backup this network drive you will need to look at another software solution capable of doing so. Some recommendations would include Carbon Copy Cloner and ChronoSync.

Another thing you can do is purchase an external hard drive capable of RAID 1 configuration. While this won't protect you from accidentally deleting a file, it will protect you from hard drive failure.