Well assuming you have SSH access and you are somewhat comfortable with typing Linux commands (or reading a lot), I recommend my upgrade procedure, here:
At least reading my write up will give you an idea of what's involved. If you're still baffled by all of this, it may be worth your while to send it to someone from this board and pay them to fix it for you and ship it back. Another alternative is just ebaying it (please let me know if you want to sell). If you have someone fix it for you, I'd assume something around 2-3 hours of work at more or less $50 to $70 per hour (or no more than +/- $200 to fix it assuming it is only a HDD problem). Unless this is for a company, it probably it makes more sense to read up here and try it yourself OR to find a Unix/Linux guru friend to help out if you get lost.
As for the drive size, I recall reading (unconfirmed) this device probably doesn't support 2TB drives. You may want to double-check BEFORE you start trying to fix it - just to be sure. It is possible that it could work, but the NAS device might not be able to access the entire 2TB of the drive. Can someone else that has actually done an HDD upgrade confirm this, PLEASE?
That's it for now. Let us know what happens. Don't forget to have fun. It's a great way to learn about Linux (especially coming from a Windows world).