I don't even know if a SATA-to-USB-to-IDE connection is even possible. At best, it's much more likely to cause problems than a direct connection.
If you search a bit, I'm sure you'll find a photo article on the web that will show you how to disassemble the WD case. I've pulled apart both Seagate and WD cases for clients who have had failed drives or failed USB controllers. Come to think of it, it's usually the USB controllers in the cases.
Commerically-built USB drives (like the WD or Seagate drives) are not known for their reliability. They tend to run hotter than internal hard drives (no fans and no space), their USB controllers are a failure point, and they tend to get jarred or dropped, leading to disk failure.
Disk warranties aren't terribly useful anyway. It costs way too much to ship the failed disk. And you get back a "refurbished disk", which I'm not sure if I'd trust, anyway. My last hard disk was purchased two weeks ago: $55 for a Terabyte disk. It's getting to the point where it's hardly worth bothering with the warranty.
My recommendation stands: Insert the disk into the computer case, hooking it up to a PCI or PCI-E SATA controller card. Those are as cheap as $10-$25 and they've worked fine for me on many computers.