I'm not sure whether this should have been posted on a different forum, but it came up after I imported my 3 500Gb external drives into the LR2 library.
A few days later, my WinXP computer decided to rename two of my external drives, resulting in question marks on zillions of images.
I spent a long time trying to redirect LightRoom to find the correct folders, but the process was complicated because other drives now had the drive letters that the drives in question had used.
Conferring with a friend who is more computer-savvy than I, I found that it would have been possible, and much easier, to rename the drives that had changed:
1: Right-click on the My Computer icon.
2: Click on Manage.
3: Click on Storage>Disk Management to bring up a graphic of your drives.
4: Find the drive in question (whose letter has been changed). Check that no other drive now has its old letter. If there is one, its drive letter must be changed first or you won't be able to correct the situation.
5: Change the drive letter by right-clicking on the graphic of the drive involved. Then click on Change Drive Letter. A dialogue box will open that has a dropdown menu containing free drive letters. Choose the correct one for your drive, and you're done, without having to redirect LightRoom! It takes about 10 seconds to do.
I've been using XP since it came out, and thought I knew how to use all of the system-related tools, but nowhere did I learn this!
I didn't either Walter until I had exactly the same issue with 1.4. I have quite a number of external drives (archives to 2000) and didn't have my drives named. I asked the question here and on the PC forum at drpreview and got the same answer--name my drives. That's solved my problem and your little 'tute' should answer it for anyone that has the same issue (I actually used a bit differnent method, but it comes out to the same).
Its likely that many will eventually have more than one external drive--so it makes good sense to start out doing this with even the first external drive (I also have each of my drives marked so I can easily find the correct drive when i want to work with older files).