the new location.  I believe this should be always true, so presumably
with all modern disk drives a write error should mean something very
serious has happend. 

Not quite that simple.

If you write a block aligned size the same size as the physical media
block size maybe this is true. If you write a sector on a device with
physical sector size larger than logical block size (as allowed by say
ATA7) then it's less clear what happens. I don't know if the drive
firmware implements multiple "tails" in this case.

On a read error it is worth trying the other parts of the I/O.

Alan