Hi, this is a somewhat strange case. Well, it's nothing too special if the drive clicks (mechanical defect -> bin it), or if it is not recognized at all (system pretending as if no drive is there).
BUT ... this is not the case here. Motor WORKS. And system clearly does indicate that a drive is there, but outputs lots of gibberish instead of the proper model+fw name. This happened from one day to another here on Linux. Kernel gave up after madly cycling through every possible UDMA mode, and eventually went down to PIO0 (!!). It was the last time when my partitions showed up...

As it's a Hitachi, I know the firmware chip (25P05VP) MUST be swapped, should I decide to buy a replacement PCB.

Besides, I remember that these issues will also happen if the jumper(s) is/are set wrongly. But I know that I have put them correctly (one leftmost, one rightmost, with two rows of 2 pins each free in between).
And, out of curiosity: have you pros ever managed to fix such drive without swapping PCB/FW?

Could be the sata or power cable. Try re-seating both cables or replace them.

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Is this the problem?

Bad model number due to stuck bit in IDE interface:
http://www.hddoracle.com/viewtopic.php?f=59&t=147&p=262

If so, then try disconnecting and reseating the IDE cable at both ends.

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A backup a day keeps DR away.

@fzabkar Thanks. Had even two replacement cables spare, and tried to connect the drive, but unfortunately this did not fix the problem at all.

Plus, I have two almost identical drives of this type, the other still working 100% perfectly.
That the cables can't be at fault, I learned when I used the very same old cable with the good drive, and this bad one. No matter if master/slave, or which channel or even at my expansion card, "good" drive always showed up as it was supposed to.

Whilst the bad drive indicates that it's there (though displaying garbage) on BIOS, it must be noted that my expansion card can't recognize this drive at all. But again, BIOS indicates that there must still be some life in the old dog

I have managed to resolve a similar issue after changing the PCB in HDT725025VLAT80 which is a 250 Gig drive and yours is a 500 gb drive. PCB is almost similar in both. I think there are great chances of resolving this issue with change of PCB.
Good Luck.

Thanks Rob. Well, in your case, it was mandatory to change the 8-leg firmware chip as well, wasn't it?

YAY. THAT "FIXED" IT. Thank you.
Guess what, I'm currently recovering my data from that drive!!

Boy, I'm so happy that worked out.
To be fair though, fzabkar deserves a big "thank you" as well, as he had the same thought before you; he simply explained it in a more technical kind of way (DD4, DD15...) plus output of 'debug' (wow, ain't hackers cool?! <g> )

But for real, I took a pair of precision (!!) pliers and bent pins around by fractions of inches, hoping it would do something good...and it actually did! It only worked with the third cable I tried (Device I/O error from hdparm with the first two), and only on the built-in mainboard connector, but heck, it worked!!
However, I can nothing but agree to the guy who spoke up in that usenet post that fzabkar linked to above: this is no longer a safe place to store valuable data, so I might get that drive replaced very soon

Well, the ugly thing is that a used replacement drive will be way cheaper than a PCB + the cost to swap the FW chip (I can't take the risk, as I don't own such a quality hot-air gun to ensure nothing vital would get damaged in the process...)

I would examine the IDE pins and look for cracked solder joints at the PCB. If you have a record of the "gibberish", then we could tell you exactly which pin to focus on.

_________________
A backup a day keeps DR away.