+/- 10% is considered safe. In your case there's more than that, so, if you care about your components, swap the PSU or repair it.

If I come across those voltage, PSU is out. I'll sell it for cheap with the note the voltages are a bit too much off.
Next thing I'll pop in a more recent more decent PSU with AT/ATX cable and call it a day.

Those voltages you are taking, are not good but won't break things: undervolt is not too harmful and the 5v runs in spec. But the capacity is clearly not balanced for your setup, this PSU needs more 12v load.

Maybe with 4x HDD the balance comes closer to spec. But then again: I wouldn't be surprised if the PSU is lightweight and has tiny cooling fins and/or missing components inside. If so: That's a bad thing, showing the pus is oriented lower than low budget. A dime saved is a dime earned they would have said back then.

I have a PSU with the same problem, outputing 9V instead of 12V. Close examination shows that one of the capacitors has burst, and I assume this is causing it.

@OP
This is a very bad PSU.
It is limiting the 12V output to prevent the +5V going too high.
Recapping does not change this behavior, it is caused by bad design.
Replacing it with a good one is the only option.

@Errius
Yours suffers from the same problem.
In addition, the ripple will probably be excessive.
If you want your HDD not dying soon, then replacement is the right option.

@waterbeesje
Excessive undervolting of the 12V rail can be very harmful, as it puts a lot of strain on DC-DC converters which these are not designed for. They have to run way faster than designed for to pump the energy.

I won't be wondering at all if the PSUs being described in this thread are FSP ones, as the behaviour described is typical for FSP PSUs and many other nonames.
I know many people here who did not do actual test bench examinations are FSP fanboys, so I know that there will probably some angry reactions on what I wrote.
My personal advice is to use Seasonic, as I never have encountered any PSU of these which fails the ATX crossloading specs like the PSUs that are being discussed here. By using a Seasonic, you just cannot do it wrong.
There are other PSUs that are good like Enermax and Bestec, too, but these are either rare/hard-to-obtain or of brands that have good and bad PSUs, so one needs to do a test bench examination before deciding whether to use or discard that particular PSU.

I think the two hard drives aren't a big enough load for the +5V rail, you can add a dummy load (power resistors, or car headlight bulb) or even a motherboard, as the voltages aren't that much out of spec, and check again.

retardware wrote on 2021-11-17, 11:31:

My personal advice is to use Seasonic, as I never have encountered any PSU of these which fails the ATX crossloading specs like the PSUs that are being discussed here. By using a Seasonic, you just cannot do it wrong.
There are other PSUs that are good like Enermax and Bestec, too, but these are either rare/hard-to-obtain or of brands that have good and bad PSUs, so one needs to do a test bench examination before deciding whether to use or discard that particular PSU.

Considering this is about AT PSUs... Does seasonic still have any AT PSU's for sale?

weedeewee wrote on 2021-11-17, 12:44:

Considering this is about AT PSUs... Does seasonic still have any AT PSU's for sale?

Used AT power supplies are most period-correct, and imho there is nothing to object against using these if these are technically OK and conform to the ATX specs.
I have had lots of AT and ATX PSUs on my test bench, and the general quality of the particular manufacturers is the same no matter which kind of PSU.

@Retardware, I didn't know FSP had these problems. I have two of the brand, of which one is fairly new (sata connectors and p4, 24p ATX main) and an older one (only miles and 20p ATX main). They both seem to give voltage almost exactly on spec. Did you note down the type numbers?

@OP I'd get rid of the PSU any way. What kind of system do you use any way? I could imagine you would be better off getting a quality modern PSU and at-atx cable. Easy to find, at least ten years no bad caps.. and for AT anything that delivers 18A on the +5v should do. Not to mention overcurrent protection.