You don't need to remove your hard drives from the case to delete all the files — that's like cutting down a tree to get a cat out of it.
If you're having permissions problems deleting files, then su to root to do the command. Since the files on the drive are on the default share, the user and group for the directories and files are probably user=www-data and group=www-data. When you ssh, you're using a different account and don't have permission to delete the directories and files owned by the user www-data.
So, su to root, then issue the delete command you tried — BE CAREFUL — if you rm -R *.* in the wrong directory as root, you can delete the entire filesystem and have a dead MyBook.