If the system is reporting it as in location 0, but you can't see it, it sounds like it is not formatted and perhaps not even partitioned (but don't take my word on that- get confirmation from someone else). If it's your backup and you are sure you didn't blank it by accident, I'm stumped.

Try Control Panel (XP view, not Classic), Performance & Maintenance/Administrative Tools/Computer Management.

In the new window, choose disc management. The upper right window should list all your hard drives, internal and external. Does this list your Maxtor external? What does it say about it? Layout, Type, File System...etc.