That just happened to a friend of mine, 500Gb WD external drive, 380 Gb worth of photos and music.
Fortunately for him most of it is still on his computer but he's lost some data forever.
I suggested that instead of a ready built all-in-one external drive he get a couple of decent external HDD caddies and a pair of 500Gb disks to put in them, then back up to both.
I've said it here a hundred times - NEVER rely on a hard disk as a reliable media/data backup, it's a mechanical device and mechanical devices are prone to failure.
Sorry to hear about your misfortune Zed
As feckit said, your stuff can be recovered but it costs and arm and a leg.
Specialist recovery firms will usually take out your hard disk platter(s) and mount them in a specialist caddy to extract data from it (them).