Just spoken to seagate support, who say the drive with this serial number does not have any firmware problems, so all they can do is pass on to data recovery, minimum cost £470 +vat thanks seagate. Just ordered some hardware for some diy tinkering, I'll let you know how I get on.
My ST3500820AS SD81 drive has just stopped working with no signs of impending failure. Not recognised by BIOS, sounds Ok and is spinning up. Could be this firmware problem ? I've seen a vid on YouTube using a modded phone cable, might try this. Any tips ?