Antoine:
We can all go into semantics and use different adjectives to describe
the problems we have with our computers. My idea of a "dead" hard
drive is one that does not spin-up and eventually become unrecognizable
by the system BIOS.

The four "dead" hard drives you said you worked on were not
actually dead,they were "defective". A defective hard drive can
still work intermittently and if you are lucky you might be able to
perform some data recovery tasks.

Also, I am amazed that you would think "fiddling cables or BIOS
settings" is out of bounds when troubleshooting hard drive issues. I
don't know what type of training you have but, if on first boot the
system BIOS tells you that the hard drive is unrecognizable, are you
not going to look at BIOS settings to make certain the correct drive is
selected, and make necessary changes within the BIOS to correct that
issue? And won't you look at your data cables to make sure they are
connected properly because, good connections play an integral role in
data transfer inside the box?

That is exactly what is expected of any good technician.

If you do not know what I am talking about, let me give you a brief
lesson about data cables and the importance of proper connections.

There is certain firmware placed on the controller boards on those hard
drives. The BIOS is located on the system board inside the box, and the
only communication link between the hard drive and the system board is
through the data cable. Now, isn't it logical that a person who knows
about fixing computers would look under the hood to see if there is
continuity between the hard drive and the IDE controller on the system
board to find out whether or not the cables/connection might be
problematic thus; those BIOS messages indicating hard drive issues?

And finally, if you look at the last paragraph in my initial response
you will see I mentioned "eventuality", and read carefully because
you might not see it if you are unfamiliar with repartitioning /
formatting, bad sectors and recovery of lost allocation units.