my wallet has a magnet, my tank bag has magnets, never had a problem with cards....
well ok I have a tendency to kill hotel keys, but I can do that without them being anywhere near my wallet or cellphone.
if you really want to know if a magnetic tank bag would kills a HDD, ask IT. have them grab an old external HDD, toss a few files on it, toss it in your bag, and forget about it for a week...pull it back out, see they're still there.
Considering how strong the magnets are INSIDE of a HDD, I suspect you'll probably be fine, but you'll only know if you science that bitch and find out.