Currently Being ModeratedFeb 20, 2012 3:47 PM (in response to Meidas)
Apple does not officially support Time Machine backups to a hard drive at the USB port of the AirPort Extreme.
Some users try this anyway...and some actually seem to make this work. Some have some limited success and some have no success.
Having tried this many times over the past few years, I remain in the "no success" category.
I do not recommend this to users. But, if you want to try it anyway, I would suggest that you also have a secondary backup plan in place to protect your important data. There is no help if you have corrupted backups with this unsupported method....which is likely.
Not only will you need a hard drive with its own power supply, you will likely need a powered USB hub as well for the drive to work at the AirPort Extreme.