External USB drive connected to Airport Extreme doesn't show up in Airport Utility

Solution 1:

I had the same problem with my Western Digital USB drive. I re-formatted it as Mac OS Extended (Journaled, not case-sensitive, not encrypted) and it worked.

Solution 2:

I would probably reformat the drive as journaled HFS+ but not Case Sensitive. Encryption is fine, but Case Sensitive is a pain in the arse, especially because the OS X is generally not case specific.

You plugged the USB drive into the AirPort Extreme, but did you use the AirPort Utility to configure everything to work together?

The rightmost tab/button is where you configure attached storage. You'll need to set the password security for either plain visibility when you useFinder to log into the AirPort Extreme, or if the hard drive will require its own password, independent of the device password.

Whole the setup parameters are fairly straight-forward, I may have missed one or two checkboxes. I'm doing this from memory.

Good luck!