I have a couple of old disks acting up after playing around with diagnostic tools, primarly HDAT2.

Fujitsu MHY2120BH (fw 0085000B)
2.5" - 5400rpm - 120GB - SATA

Seagate "Barracuda 7200.12" ST31000524AS (fw JC45)
3.5" - 7200rpm - 1TB - SATA

Both report reallocated sectors in SMART, react really slow, and as soon as they have spun up, theres heavy seeking all the time.
Since I'm a stubborn fool, and for a number of reasons, I suspect this is somewhat erroneous and I would like to try to fix this.
I've had success in seemingly resetting and re-evaluating supposedly unrecoverable reallocated sectors in other disks, but these two seem to me to be in a software-induced state I can't seem to retrieve them from.

So, it's been suggested to me by other users of this forum that I should try accessing the serial port terminal which may or may not let me "convince" these two into behaving properly... that is, if this is all bogus, as I suspect it is.

My current computer's motherboard does have an RS232 serial port header and I do have an cable to route that to an external DE-9 like in the old PCs. I've verified it works with an old router and some miscelaneous devices.
But, what I've been reading around is that HDD serial ports use the TTL variant of RS-232, instead, and regular 12V(?) version will burn them.
There seem to be converters that will adapt a regular port into TTL, but there are also direct to RS232TTL USB adapters.
Will any of those work? Any suggestions on what to buy?

Any other info on how to begin with this? I'm guessing this works with PuTTY? I'm not an expert but I've used it a couple of times. Any other software preferable?

Are you really willing to go all that way just to semi-fix 2 hdd's?

Anyway, buy a usb rs232ttl and use hyperterminal.
It will be boring, trust me.

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Well, if you must know, I'm really poor ATM. I connect to the internet by USB tethering an old 3G phone which was mid-low-end in 2015, and 3G is going to be taken down here in Spain very soon, so I'll have to get a new phone, and I already don't have cash for that, much less new HDDs. If I can try to fix these on the cheap, I'm up for it.

Will any RS-232 TTL interface do, or are there any specifics I should look for?

Any RS-232 will do, providing you have the knowledge.

You will waste money for nothing.
There is a reason why pc3k and other tools are expensive... It's because only a terminal is usually not enough (even if you have all the knowledge) to fix a hdd.

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Well, my idea is to see if there's any way to kind of force a factory reset of sorts and then let the disk re-evaluate things to see if there is, indeed, any actual problem, or the whole thing is bogus.
If that doesn't work, then it's not a big loss, I guess. I'll keep the RS232ttl for when I need it again or sell it maybe.

I'm assuming those tools you mentinon are mainly geared towards data recovery, which I'm not doing here. And yes, I understand that often that task involves restoring a drive to a certain level of functionality for a chance to extract the data, and I suppose that this involves particular access procedures that can't be properly done with consumer interfaces, which is the reason for the special equipment. In these cases, the value is in the data to be recovered, which is transfered to fresh media, and, when done, the drives are usually discarded as faulty or suspect/unreliable.
In my case, there's nothing in these disks, and I just want to try to use the drives again. Surely I won't trust them with anything important. I'll make sure not to put anything there that it'll be a problem if it randomly goes south, and only after some stress testing.
But, again, given the circumstances of how these disks ended up like this, I'm inclined to think that the problem is logical and not physical.