I keep 2 external USB backups of some critical stuff. I went to add
some more material to them the other day and one of them suddenly
stopped working. (External WD). Lights on, nobody's home. Windows
doesn't even recognize something is plugged in.

As bad as that might be, like I said, I have another identical copy. So
I fire up teracopy and tell it to copy one drive to the replacement that
WD sent me in advance. 794.99GB with time fluctuations between 3 and 25
days for an estimate. USB to USB copy I don't have firewire or
esata on this particular box, but the drives support all 3.