Laptop no longer sees external HD attached to Airport Extreme

[Beccad]

On Saturday, I set up an Airport Extreme and networked my iMac and my Windows 7 laptop. I attached an HP110 printer and an external HD (formatted with 2 partitions, one for Windows, one for Mac OS). After a little bit of fiddling, all was great -- I could access the external hard drive and all shared files from both computers. I apparently patted myself on the back too quickly. Last night, the backup failed on the Windows laptop. The external HD is no longer visible/accessible from the Windows laptop. It is still accessible from the iMac. I've rebooted; still not there.

Any thoughts on why the drive disappeared? Any suggestions on what to look at to isolate the problem?

Thanks!

Becca

iMac 21.5" 3.06GHz Intel Core i3
Samsung QX410, Windows 7, i5
iPad 16g 3g
iPhone 4
iBook G4 12"