External Hard Drive Letter/Path

How do I manually assign hard drive letters?

Problem and Information: Offline stand alone for security purposes; Windows XP Professional; use 2 external HD and thumbdrives - primarily photographic work; XP automatically assigns drive path letters; sometimes changes letters (because all drives are not always turned on?); Adobe Lighroom Catalog will then not follow the new letter path to load photos I have to manually browse and re-find them for the catalog; it would be much easier to simply change the drive path letter back to the orginal but I can't find information on doing this; can someone help?