I plugged in a GoFlex 1.5 TB drive to the USB port on the back. Then I went to the Airport Utility and the AEBS to edit. Under Disk tab, I expected to see the drive, but it never sees it and there does not appear to have a way in the AEBS software to add the drive.
Two possibilities......
1) The AirPort Extreme USB port will only support a drive that is formatted for Mac in Mac OS Extended (Journaled), or in a Windows format of FAT16 or FAT32.
Unless you purchased a drive that specifically states that it is formatted for Mac, you likely have drive that was formatted for one of the more common Windows formats of NTFS or exFAT, neither of which are supported by the AirPort Extreme. If that is the case, then you will need to reformat the drive using Disk Utility on your Mac first, before you connect it to the USB port on the AirPort
2) The AirPort Extreme USB port, actually it is USB 2.0....not the newer and faster USB 3.0.....is not a high power port, like you would typically find on a computer. So, if you have the drive formatted in Mac OS Extended (Journaled)....strongly recommended....and the drive still will not show up when you check in AirPort Utility, you may need use a powered USB hub.....even if your drive has its own power supply.
I unplugged it and brought it back up to my office and plugged it into my Macbook Air and it is running just fine.
The Mac supports more formats than the AirPort Extreme, so if your drive is set up as NTFS or exFAT, the Mac can read the drive. The AirPort Extreme cannot.
Finally, some drives that are formatted correctly and used with a powered USB hub just don't work at the USB port of the AirPort Extreme for unknown reasons. Apple does not provide a compatibility list, so that makes things difficult. About all that you can do in this case is try a drive from another manufacturer.