OK, sorry, I am looking the board at a different angle (my top is where Q9 is readable, -45deg on the above photo), so the base would be top RH corner.
Q9 reads "TL" (I think, it is tiny even under the magnifier).

Will be back in 1' with the measurements.

Control pin on the old PCB: 3.73V and 3.43 on the new PCB.
Base pin resistance: 73K on the old PCB, 81,5K on the new one (nothing on the 200/1K range, had to go up to 200K to get a reading).
Diode tests are the same for both boards: Q9 control (red) -> 5V (black) OK, opposite not OK. Q9 control (black) 3.3V (red) OK, opposite not OK.

Sorry, I should have asked you to test the resistance between the 3.25V test point and ground, not between base and ground (no power).

In any case, if the base pin is sitting at +3.73V and the +3.3V test point is really sitting at +0.64V, then Q9 is faulty, assuming that Q9 is indeed an NPN BJT. Could you confirm these voltages once again (with power)?

Also, could you tell me the reading on the diode range when you place your red and black probes on the base pin and 3.3V test point, respectively? If Q9 is an NPN BJT, then you should get a reading of around 0.600 (no power).

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A backup a day keeps DR away.

Resistance from 3.25V test point to ground: new 2,58K, old 2,67K

Double confirmed that Q9 reads 3.73V on its base pin and 0,64V on the 3.25V test point.

Diode reading with red on 3.25V test point and black on Q9 base pin is 1783 for the old board and 1951 for the new one (no reading if I swap)

Sorry, I will now *have* to go to sleep, it's 3:15 AM here

Will check any further info you post tomorrow morning and continue from there.

Again, thank you for your time and great help; I really hope it's Q9, so that I can swap the sucker and get done with this!

See you tomorrow.

BTW, this is how I think the circuit is configured:

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A backup a day keeps DR away.

Just finished with the soldering work:

Picked up both Q9s from the boards with a heat gun and then soldered the one from the new board to the old one.

Here are the results:





(just found out how much I suck at soldering tiny stuff )

Although it's choppy, the legs look clear to me for bridges, do you think I should fire 'er up and see what happens?

I had intended to take additional measurements before doing any soldering. If the SDRAM or MCU is responsible for the failure of Q9, then your effort will have been wasted.

To this end I would measure the resistance between the 3.3V test point and ground. If the resistance is very low, then that would point to an additional problem. Do this before powering up the PCB. If the resistances of the donor and patient PCBs are similar, then you should be good to go. Just power up the PCB on its own before installing it on the drive.

BTW, I hope you paid attention to the orientation of Q9. The dot on the package is pin #1. The orientation looks OK, but the PCB marking is unfamiliar to me.

_________________
A backup a day keeps DR away.

Hmm, I wish you had said that earlier; I though that yesterday's conversation was conclusive

The resistance is 3.57K on the new board and 3.46K on the old one (which now has no Q9). How does that sound?

As for the orientation of Q9, yes, I paid attention and I am certain that it is correct.

What are the odds here? Is there any chance this might work, or should I leave it as it is and take both boards to an expert?

It looks good to me. As for experts, you've already done a lot more than 99% of the professionals would do. A Toshiba HDD that has a dead PCB without a discrete "ROM" would be considered unrecoverable by nearly everyone.

Best of luck.

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A backup a day keeps DR away.

Really?? To tell you the truth, I was doing all this thinking that if something went wrong, there would always be the "expert" option.

Unfortunately, this data is critical to my business and I feel truly stupid for not having a backup for the first time in my life (actually, I did keep daily db backups, but on the same disk; stupid, I know).

So, do you really think I should plug it in? (without the disk, of course, just the board)

So, what should I measure after I connect the power?

Also, are you sure the soldering work looks OK? I am pretty worried about this, too. If it is not, would it mean that it might produce further damage than there already is?

Measure the +3.3V test point. If that is OK, then you can try the PCB on the drive.

As for the soldering work, just confirm that the resistance between the pins is not zero ohms (except for those pins that are meant to be connected, of course).

If you are uncomfortable with this task, you might like to consider the following service:

http://www.onepcbsolution.com/largerepairs.html

There would be no guarantee of success, though.

IMO, if the +3.3V rail comes up and the drive still does not spin, then the MCU would be a serious suspect, in which case you would need to find someone who could recover your drive in the absence of a PCB. However, before you do this, I would examine the drive's serial terminal output, as explained by Spildit.

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A backup a day keeps DR away.

So, the pins that are meant to be connected are the ones facing each other, I suppose?

Actually, after measuring the resistance between all the in pairs and although I get many 0s between some adjacent and opposite pairs, the results are exactly the same as the ones I get on the new board (without the Q9 attached). Does that sound OK?

The 4 pins nearest the dot (2 on one side and 2 on the other) appear to be connected on the PCB. The other two pins appear to be unique. A transistor has 3 unique electrical connections. The reason for the multiple pins is current sharing.

In short, it all sounds OK to me.

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A backup a day keeps DR away.

OK, I will try it in around an hour and report back...

Just did the power test and the 3.25V test point looks good now
Anything else to test before I give it a shot with the disk connected?

Looks good.

Best of luck.

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A backup a day keeps DR away.

MY GOD, IT FREAKING WORKS!

Man, I cannot even think of a way to express my gratitude!

This has been a true lesson, both for the electronics/soldering experience and the daily backup thing.

THANK YOU so much for your time and knowledge!