With the release of OS X 10.9 Mavericks, Apple no longer supports backing up to a hard drive connected to an AirPort Extreme with Time Machine. This was a huge disappointment for me, as I had just purchased an 802.11n AirPort Extreme and portable hard drive for my backups when Mavericks was released, only to discover that Apple now requires a Time Capsule only to perform backups. Luckily, there is an easy workaround that will allow you back up to a portable hard drive that is connected your AirPort Extreme with Time Machine. Check out the instructions after the break!
Open a Terminal and run:
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
Reboot the machine.
Open Finder, and browse to the hard disk that is plugged into your AirPort Extreme to mount it.
Open the Time Machine Preferences, and attempt to select the disk that is connected to your AirPort Extreme.
The disk connected to your AirPort Extreme should now be visible in Time Machine Preferences!
So far, this has worked for me with both a 2009 Mac Mini and a Mid-2012 MacBook Air, both running OS X Mavericks. I've heard some say that using unsupported volumes is risky, but the truth is, this shouldn't be your only backup solution. I recommend pairing Time Machine with Crashplan, or some other backup system. If this process doesn't work for you, let us know in the comments!