Portable HDD's don't normally have drive letters assigned to them and thus may not always have the same drive letter/path if they are removed or, in your case superceded.
Right click on My Computer and select Manage. Select Disk Management in the left hand pane. Right click on your external HDD and select Change Drive Letter and Paths...
Assign it a drive letter of F: and all should be good.
A note - make sure no other drive is using F:, if so you may need to remove it first.
Darren, April 2007