JRiver Media Center does not alter the drive letters. The operating system may forget the previously used drive letter if you have the external drive disconnect when you connect some other device like a camera or a flash memory drive and the other device gets the same drive letter.
You can change the drive letters with Windows Disk Management tool (run: DISKMGMT.MSC). It might be good to change the external hard drive use the highest free drive letter (X, Y, Z,...). Then Windows will be more likely to always use the same drive letter.
When the Windows drive letter has changed you can adjust the file paths inside the library with MC's tools. There is no need to clear the library. For example, you can change all
D:\ strings to
E:\ strings in the filenames. The playlists that are inside the library will be adjusted accordingly. You can find links to detailed instructions here:
http://yabb.jriver.com/interact/index.php?topic=57922.msg392381#msg392381To fix the problem you could change the drive letter back to D: and then restore a library backup file that precedes the incidence (File > Library > Restore Library). Once the library works again you can change the drive letter e.g. to X: and fix the library files to point to X:.
EDIT: or just change the drive letter directly to X: or similar and fix the restored library to use it.
Alternatively you could fix the drive letters inside the exported playlists, but you would need to use a find & replace tool in some external text editor.
In the future when you export a playlist you can enable the "relative" option and export to the root folder of external your drive. Then the directory paths inside the playlist start from the next folder level, not from the drive letter.