Jeff Urban Inscribed thus:

Yes, the ground thingie(s). I would like to think this was caused more
by something connected that shouldn't be rather than the other way
around. However now we know that what fried was a protection diode.
Could it have been across what was suppose dto be 3.3V but was 5V ?
What would they have in there, a 3.9V zener ?

If I did the math right, feeding a 3.3V device with 5V would result in
somewhere aroung 2.3 X the power dissipation is it's analog. Digital
may be a bit different, I would think lower, by that I mean a slightly
smaller increase in dissipation. This because for the most part heat
in a digital device is from the saturation voltage of the
semiconductors and the capacitive load on anything that switches.What
this all means I think is thast a 3.3V device wil actually work on 5V
for a while, how long depends on alot of things. If the components
just barely meet spec, minutes or hours, but if you happen to have a
really superior specimen, who knows how long it may work, years
even.

If a similar diode exists for the 12V suply, it is probably 15V, which
means one volt will not bother it. In fact driving a motor, it might
just be fine with an extra volt, but the next standard zener value up
from 3.3 is 3.9. One volt is too much.

So if somehow a negative 1V somehow got on the commons, this would
explain it. But then how would that thappen ? Concievably if it were
an open circuit and chassis ground made that connection, even though
it's a bit far fetched as it assumes quite a bit of resistance in that
ground. It could happen though, with the uPrecessor drawing alot of
amps during bootup. Something like this may actually require a chain
of partial failures, just to make that one volt.

I agree that this is an intersting subject because the actual cause
seems to be elusive. If we don't know why it happened this time there
is no way in hell of keeping it from happening again.

J

I can see where you are going, but consider this...
A PC PSU has three major voltage outputs +12, +5, and +3.3. These are
the Yellow, Red and Orange wires respectively. Black is common to all
three. The Black wires, are normally common to the chassis. That is
the negative pole of all three are bonded to the case. The case is also
ground and would normally have an earth connected to it. At least in
the UK.

Now if you place a volt meter between the case and any output you will
read an appropriate voltage, 12, 5 or 3.3 volts.

Now do the same but connecting the voltmeter between the 12v and 3.3v
outputs. Depending upon which way round the meter test leads are you
will read + or - 8.7 volts. Do the same with any pair and you will see
a voltage equal to the difference between the pair.

This is exactly the scenario if the ground was OC on the sata drive or
if any other device had an OC ground.

--
Best Regards:
Baron.