I'm not sure if Time Machine can do this. I'm guessing yes, but I've never used it.
You can use rsync from the terminal (or run it as a cron job) to backup your drive. It would go something like this:
rsync -a --delete /thing/to/backup/ /path/to/backup/storage
or:
rsync -a --delete /Volumes/MEDIA_DRIVE/ /Volumes/BACKUP_DRIVE
That would create a mirror of MEDIA_DRIVE on BACKUP_DRIVE. The trailing slash after MEDIA_DRIVE and lack of one after BACKUP_DRIVE are important.
-a means "archive," and tells rsync to maintain file attributes. --delete tells rsync to delete anything on the receiving end that isn't on the sending end. This keeps the drives "sync'd," but it will also destroy anything it deletes. Be careful with that. If you want to test it first (you should), in a terminal you can do:
rsync -nav --delete --progress /Volumes/MEDIA_DRIVE/ /Volumes/BACKUP_DRIVE
The -n means, "dry run." rsync won't do anything when that argument is passed. The -v and --progress (those might be redundant here) will show you everything that is (or would) happen. Use this to verify what would get copied, deleted, and so on prior to actually running it.
rsync is pretty powerful. You can back things up across your local network or the internet. You can be precise about what you want/don't want copied, and how it should handle those things. It might be helpful to check out the man page ("man rsync") before using it.
Edit: Apparently Time Machine can do this if you just go into its preferences and allow the other external drive to be backed-up.
https://discussions.apple.com/thread/3366161?start=0&tstart=0
However, it seems that TM can only read HFS drives.
