Hello Everyone,
First of all I would like to thank for everyone for the valuable information and advise!
Hopefully things are clearer now!
Here are some answers and other info, in no specific order.
The PC is about 10 years old, I built it with an ASUS M2n-SLI MB with AMD dual core processor, and I think with an ATI graphics card. It is still running WXP Professional.
The board sees other good SATA drives.
I heard from a friend of my son, who did tech support at a surplus store, that they did swap control boards on Seagate HDs, and it worked for them (no data on how well, how long, e.t.c.)
The drives I swapped the boards from have the same part number and even the production date is the same!
Did some measurements on the failed board and on the good one.
It looks like the failed board has trouble with the +5V line.
I enclose a picture of the board.
R67, at the bottom-middle, marked "000" is a resistor, or a "fuse", from the +5V input connectors, the three pins below that resistor. The resistor itself is (and measures) 0 Ohm, but if I measure at the pads, the resistance is about 80 Ohm or so. Looks like the soldering took a hit.
At the other end of R67, the internal +5V, there is a diode, D2, to the ground. That diode is shorted. As "wraper" suggested, it looks like it is a Zener Transient Voltage Suppressor. It is marked 6AA 37. from ON Semi, but could not find exact datasheet. The marking is probably just a snippet of the full part number.
Did find other SMD ON Semi ZTVSs that are meant for +5V.
As for the HD board, I plan to touch up the soldering of R67 and will try to remove the diode. I see if I can scavenge a similar diode from another hard drive that is past of its prime.
Hopefully my "SMD rework skills" are good enough for this. Luckily that diode is quite big!
The PS that failed is a Coolmax V-500 unit. It is reviewed (and thoroughly trashed) on this web-site:
<http://www.hardwaresecrets.com/coolmax-v-500-power-supply-review/>
Mine is a bit different, probably an earlier version, with a bit beefier components (I guess it did not help!).
Internally the board was marked as a 450W supply!
So far I only checked for elco ESRs (all fine), and the power transistors, those are fine too. Will dig into that later...
I will keep you posted!
Thanks again for the help!
Peter