Quick question. When you say "it can't find the external harddrive" do you mean as a scratch disk or when you try to look for files on your external hard drive?
Does your external hard drive show up on your desktop?
If we can answer these questions we can get a lot closer to a solution...let me know. Cheers!
Strange...you could try double clicking on the file on the hard drive to "force" it to open in photoshop, assuming it's a photoshop file in the first place of course. What kind of drive are you using?