I have just had to do this after moving a large number of downloads from my hdd to an external (very large) drive as I was getting low on space.
If you still have the torrent files on your PC (I keep them in a folder called "torrents" for this purpose), just double click them and, when asked where to save to, point it to the drive/folder where the files are now and, as said, it will check the folder, find the file, check it and then start seeding.
In Azureus, don't worry about the warning that some files exist and will be overwritten etc, just click "yes" or "OK".
This will only work if you haven't touched the files - renamed them, for instance.
If you have, you will have to start from the beginning and redownload the full files to a folder of your choice, alas. If you can rename them to exactly what they were called though, it should work, I believe.
It's much easier than it sounds
