A few days ago, a friend came by with his AirPort Extreme base station (AEBS) and his external USB hard disk. He was having problems getting the hard disk recognized and shared through the AEBS in a seemingly random way. The AEBS would correctly share the disk, until you would directly connect it to a Windows PC to transfer files. After that, reconnecting it to the AEBS's USB port would lead to an unmountable network share on the Mac, and an empty share on the PC (Windows XP SP2).
We thought of a formatting problem, so we first reformatted the 500GB disk as an HFS+ volume, and tried again connecting it to the AEBS. Transfers would work both ways from the PC and the Mac through the AEBS. But after connecting the disk directly to the PC and transfering files with TransMac, then plugging the disk back on the AEBS, we would get the same problem with an unmountable share (the problem wouldn't appear after connecting the disk directly to the Mac).
We restarted the base station several times using the AirPort Utility without success. Until, on a hunch, I decided to try unplugging the power plug from the AEBS, and then plugging it back in. After trying the same procedure several times, this appeared to reliably lead to the recognition and mounting of the shared hard disk.
Unfortunately, we didn't have time to test more scenarios (like reformatting the disk as FAT32), but the bottom line is: restarting the AEBS from the AirPort utility doesn't seem enough, so unplugging the power and plugging it back may be more helpful.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070628200327500