Make sure that the drive is formatted in Mac OS Extended (Journaled) to work correctly with the AirPort Extreme. Unless you purchased a drive that was specifically formatted for Macs, the drive is likely pre-formatted for use with PCs.
Use Disk Utility in your Utilities folder under Applications to format the drive if needed.
Often, it helps to use a powered USB hub with a drive...even if the drive has its own power supply....since the USB port on the Extreme is borderline as far as its power capabilities.