|Does the PC make the familiar "Hardware Connected" sound when you plug in the usb hard drive?|
If it does, but the drive isn't shown in Windows Explorer, it may simply be that Windows hasn't assigned a letter to the drive. You can do this manually:
Control Panel > Administrative Tools > Computer Management
Click "Disk Management" in the left pane.
Right -click the horizontal ribbon which represents the external drive & choose
"Change Drive Letter & Paths"
Click "Change" & choose a letter from the drop-down box.