That happened to me as I was transferring photos from my netbook to my main laptop. Reconnecting the catalog with the photos was also a royal pain. Fortunately, I found out that it's possible to fix the drive letter assignment for the external drive. This will not only reconnect the catalog with the photos, but will fix the drive letter to avoid similar issues in the future. Note: These instructions apply to Windows Vista.
- Note which drive letter Lightroom is reading your photos from. If you're not sure, find any "missing photo" then on grid view, click on the small question mark on the upper right of the photo. In the popup dialog box, you'll see at least part of the path that Lightroom is using for your photos, including the drive letter.
- Close Lightroom and any apps that are using the external drive. Better yet, exit all apps.
- Eject and disconnect any devices that are using the drive letter for your photos. Leave the external drive connected.
- In Windows Explorer, right-click on Computer or My Computer.
- Choose "Manage"
- On the screen that appears, see the leftmost column and click on "Disk Management" under "Storage".
- In the middle column, right click on the external drive and choose "Change Drive Letter and Paths"
- In the next dialog box, click on the button "Change..." then select the drive letter that Lightroom was using for your photos (see step 1 above).
- You'll get a warning from Windows that doing this may cause programs not to run correctly. Just click "Yes."
- To be safe, reboot.