Problems with Airport Extreme and External Hard Drive

I have an Airport Extreme that wirelessly connects an Apple eMac and a MacBook. By using a USB hub I am able to print from both computers. I also connect a Toshiba 320GB external hard drive to the hub, but I can't get the drive to be recognized by either computer. No icon appears on the desktop and when I try to back up my computers and have to select a target disk, my hard drive doesn't appear in the list given. When I disconnect the hard drive from the hub and connect it directly to the computers, both computers recognize the hard drive and show the icon on the desktop.

Any suggestion as to how to get the USB to recognize the hard drive?

Thanks.

Have you turned on the disk sharing feature in the Airport utility?

Thanks for the input, but.....

I have turned on the disk sharing feature in the Airport Utility. My hard drive is listed so I have selected it. Yet no hard drive icon is visible on the desktop nor is it listed in the sidebar. Plus, when I try to back up my computers to this hard drive using Carbon Copy Cloner my hard drive is not listed as a target disk. Any other suggestions. Thanks!