Why won't my Macbook Air access a network hard drive?

I have a BT Home Hub 3 and have connected a Western Digital hard drive into the Home Hub's USB port.

I have an iMac running Snow Leopard, and a Macbook Air running Lion. When I open Finder on both machines I have "bthub3" appearing under the Shared heading in the sidebar.

Clicking on the bthub3 on the iMac enables me to access the WD drive without any problems at all.

However, when I click on bthub3 on the Macbook, it shows a folder called "USB_Disk", but when I click on that I get the message "The operation can't be completed because the original item for "USB_Disk" can't be found."

Searching the Apple support pages there is a page headed "OS X Lion: If you see "The original item cannot be found" when using a remote disk" which goes on to say that I may be attempting to do something which I can't do with a remote disk."

This makes no sense to me, especially as I am can't access the hard disk from the iMac with no problems.

Any thoughts on how I can access the hard disk from the Macbook?

Thanks very much.