Which disk errors are unrepairable?

Well, for one, SMART failures are purely hardware ones, of one sort or another. If you get any read or write errors, and under some circumstances any disk I/O errors, those are hardware.

And naturally if you hear any strange noises (clicks, thumps, banging, etc.), that's definitely hardware.

 

In my experience, B-Tree errors can't be reliably repaired and of course, the aforementioned hardware errors, of course.

 

I have an external WD Mybook HDD that I keep getting I/O and R/W errors. What program can I use to make sure this is hardware problem so I can get it sent back and repaired?

I have tried Disk Utility and DiskWarrior, but neither seems to offer a hardware test feature for external HDDs.