1. Back up your library. File > Library > Backup library . Note where you put the backup zip file.
2. Get your new drive mounted under a directory. It will probably be under /Volumes somewhere.
3. Copy your music to the new drive, using the same directory structure as the original on your USB drive.
4. Disconnect the USB drive. Verify that the music is on the new drive.
5. Use the Rename, Move, and Copy tool in Update Only mode to tell MC that your files have been moved to a new directory (folder). I recommend using the Find & Replace mode of the tool, as outlined here:
http://wiki.jriver.com/index.php/Find_%26_Replace_TemplateHere's the master wiki entry on the Rename, Move, and Copy tool:
http://wiki.jriver.com/index.php/Rename,_Move,_and_Copy_FilesIf, at some point, you figure out that you've horribly screwed up the database, you can use your library backup made above to restore the database.
IMPORTANT: Make double sure that you only run the RM&C tool in UPDATE mode. That way it won't try to copy or move files. It will just update the internal database.
Good luck,
Brian.