I think something was terribly wrong with that volume. I started fsck on it and it said fs contains errors, check forced, and it's been running for an hour doing nothing. It's an empty volume so even 500G shouldn't take very long, right?
Anyhow, I'm going to leave it fscking overnight and if it's still doing nothing tomorrow, like I said, it's only 36KB of data so I'll just pull that off and wipe the file system.
Geepers, I wonder what it is?