If your drive isn't even detected anymore, it is more than likely your drive has failed.
The only way to get any data back would be to send it off to some hdd recovery service company. No idea what it would cost, though certain it won't be cheap.

I see you're in canada, you could try HDD Recovery Services in Ottawa, ON

but before you do that, make sure all the connectors are still plugged in and well seated. Unplug and plug'm back in, then try the detection again.

I'm not worried about the data, I'm just trying to keep old HDDs functional if possible. I had no idea I could cause a drive to be unreadable in BIOS with HDD software, but then again I also know that manufacturer software can write to HDD firmware so maybe that's what TestDisk did.

AirIntake wrote on 2022-02-03, 17:25:

I went to turn on my P3 500 yesterday and was greeted with an empty unbootable drive of the wrong capacity.

provide more details
this sounds like drive was already bad at this point, or your PC CMOS somehow got corrupted in magical way changing saved drive geometry and simultaneously matching valid checksum. Another option is bad IDE cable.

AirIntake wrote on 2022-02-03, 17:25:

In my effort to fix it I changed the drive geometry using a program called TestDisk by CGSecurity

No you didnt, because you cant do that on any drive without proprietary service programs. The best you could do is set HPA reducing drives capacity.
What exactly have you done/clicked?
Edit: looked at TestDisk, what you clicked affects how TestDisk interprets data, it doesnt change anything inside the Disk. Most likely your disk just died. You can try USB enclosure/bridge or another IDE cable and computer just to be sure, and recheck drive jumpers.

The drive was showing up fine in multiple computers, Windows 98, XP, and 10, but all of them showed that the 40GB drive only had 3GB and was completely empty of data.
I tried several partition recovery programs which detected the lost files, but they were unable to actually recover the files due to drive error (can't remember specifics).
TestDisk detected the same lost files but also couldn't recover them because it said there was a geometry mismatch, the heads was set to 255 and it suggested it should be 128.
I changed the heads to 128, rebooted, and the drive detected fine but TestDisk was still reported a geometry mismatch and was unable to recover files.
From there I changed the cylinders, heads, and sectors to what was printed on the HDD. This made it an 8GB drive but it still rebooted and detected fine in BIOS, but still showed a geometry mismatch in TestDisk.
That's when I modified the sector size from 512 to 2048 because I was out of ideas and rebooted. From then on the drive was not detected in any of my computer's BIOS (3 different systems, P3 500, P4 2.4, C2Q 9650 all different cables). I have double checked jumpers and they're fine, unchanged from when the drive was detected.
I really don't think the drive death just happened to coincide with the change of sector size, I think it's definitely related. Everything that was wrong with the drive I should have been able to fix with an FDISK and reformat based on previous experience until I messed with the geometry.

What is the drive's model ?
Do you hear unusual sounds when you power on the drive ?
Have you taken off the PCB and and observed/cleaned the contact pads/pins that are connecting the PCB with the spindle/voice coil ?

It's a Seagate Barracuda ATA IV Model ST340016A. It sounds normal when I power on the computer, the drive spins up as normal and stays spun up. Nothing sounds strange.
I just removed the PCB and checked the contact pins/pads and they were pristine. I cleaned them anyway but it still doesn't show in BIOS. I've had dozens of HDDs die on me in my time but a dead drive being undetectable by BIOS is a first for me.

I can confirm that the hard drive wasn't dying before I played with the geometry. I just spent all day reinstalling Windows 98 on my P3 500 on another hard drive (80GB). When I pulled that hard drive out and put it in a newer PC to image the drive, it showed up as only 60GB and was completely empty of files, just like the last hard drive.
So the initial issue seems to have something to do with the way my P3 500 is formatting hard drives. I will not be playing with geometry this time, I'm just going to format it in the newer PC, reinstall in the P3 500 and go from there.

AirIntake wrote on 2022-02-03, 22:41:

I can confirm that the hard drive wasn't dying before I played with the geometry.

earlier you wrote

AirIntake wrote:

was greeted with an empty unbootable drive of the wrong capacity.

from the horses mouth:

https://www.cgsecurity.org/wiki/Menu_Geometry
>CHS numbers change the way that TestDisk looks for partitions and calculates their sizes, etc. It does not affect the hard drive itself, unless you actually write data about lost partitions to the drive. Choosing the wrong geometry settings and then saving any lost partitions based on those faulty settings might make it harder or impossible to recover your data.

you can download Victoria https://hdd.by/Victoria/Victoria537.zip but _do_not_ run it on your main computer because wrong click might mean erasing your HDD 😀
Victoria will be able to detect all drives connected independently from bios/OS, but imo drive just died

unless you actually write data about lost partitions to the drive

Perhaps that's what I did somehow? I really don't think it's dying because another HDD just exhibited the same behaviour in relation to my P3 500 when I tried to image it and it's a different model, size, and age and I've used it recently just fine. I've imaged a half dozen Windows 98 hard drives in the last few days and I didn't have any issues with them.

I'll give Victoria a try.

AirIntake wrote on 2022-02-03, 21:24:

TestDisk detected the same lost files but also couldn't recover them because it said there was a geometry mismatch, the heads wa […]
Show full quote

TestDisk detected the same lost files but also couldn't recover them because it said there was a geometry mismatch, the heads was set to 255 and it suggested it should be 128.
I changed the heads to 128, rebooted, and the drive detected fine but TestDisk was still reported a geometry mismatch and was unable to recover files.
From there I changed the cylinders, heads, and sectors to what was printed on the HDD. This made it an 8GB drive but it still rebooted and detected fine in BIOS, but still showed a geometry mismatch in TestDisk.
That's when I modified the sector size from 512 to 2048 because I was out of ideas and rebooted. From then on the drive was not detected in any of my computer's BIOS (3 different systems, P3 500, P4 2.4, C2Q 9650 all different cables).

IF you software changed the sector size to 2048 (no Hard drive uses that, only 512 and 4096... 2048 is for CDROMs) and it was written to the drive partition table/sector headers like DrivePro and Disk Manager does to enable large HD support you messed up the HD where when using "auto"on a newer motherboard could give conflicting info (HD bios says one thing, HD platters say another) and messed it up because older machines will not understand a 2048 byte sector off an HD.
Basically now the only solution is to re-write the HD using same software and put it back at 512bytes/sector, sort of like if you use Drive Pro to enable large disk overlay -> use Drive pro to remove it.
Sure other apps may be able to do it but I would try the app you used first.

Take it from someone who has screwed up some stuff: Never believe one piece of software if it tells you something is wrong. Always get a second opinion (sounds like a doctor thing right ?)
Testdisk is free and not saying it is not good but my go to Imaging software is Acronis and until a few years ago was probably one of the best and still might be...

Horun wrote on 2022-02-04, 02:16:

IF you software changed the sector size to 2048 (no Hard drive uses that, only 512 and 4096... 2048 is for CDROMs) and it was written to the drive partition table/sector headers like DrivePro and Disk Manager does to enable large HD support you messed up the HD where when using "auto"on a newer motherboard could give conflicting info (HD bios says one thing, HD platters say another) and messed it up because older machines will not understand a 2048 byte sector off an HD.

wait, are you saying bios AUTO reads partition table? 😮 why would it do that when ata identify drive returns all the info bios would ever need.

Horun wrote on 2022-02-04, 02:16:

Basically now the only solution is to re-write the HD using same software and put it back at 512bytes/sector, sort of like if you use Drive Pro to enable large disk overlay -> use Drive pro to remove it.
Sure other apps may be able to do it but I would try the app you used first.

if the above is correct then Victoria will be able to do it no problem. Other option is manually setting CHS to 16383/16/63 and fdisk to erase any partitions or seatools and zeroing drive.

Horun wrote on 2022-02-04, 02:16:

Testdisk is free and not saying it is not good but my go to Imaging software is Acronis and until a few years ago was probably one of the best and still might be...

I dont see anything wrong with Testdisk, other than spartan interface 😀

Changing the sector size was(/is) fairly common on scsi drives.
I don't recall it ever being done, needed or even possible on IDE drives

TestDisk and Seatools can't detect the drive. Victoria requires Windows XP+ but I can't get past BIOS (or even into BIOS) if the bad hard drive is installed because on my XP+ computers BIOS HDD detection is always enabled and any attempt at BIOS detection causes a freeze. I'll have to make an XP install for my P3 500 in order to test Victoria.

You cant trie HDAT 2 https://www.hdat2.com/, can be booted from cdrom and only the hdd.

I think HDAT2 has detected the HDD as a CDROM drive! I'm reading through HDAT2's manual though and I'm not sure what to do next.

That's really strange, never seen that, can you post a photo?

Sure, I just have to finish installing XP that I'm going to use for Victoria.

Hoping wrote on 2022-02-04, 22:02:

That's really strange, never seen that, can you post a photo?

Here you go

rasz_pl wrote on 2022-02-04, 11:21:

Horun wrote on 2022-02-04, 02:16:

IF you software changed the sector size to 2048 (no Hard drive uses that, only 512 and 4096... 2048 is for CDROMs) and it was written to the drive partition table/sector headers like DrivePro and Disk Manager does to enable large HD support you messed up the HD where when using "auto"on a newer motherboard could give conflicting info (HD bios says one thing, HD platters say another) and messed it up because older machines will not understand a 2048 byte sector off an HD.

wait, are you saying bios AUTO reads partition table? 😮 why would it do that when ata identify drive returns all the info bios would ever need.

No I did not say that and I was trying to keep it a simple explanation.
If you use software (like a DDO, like Drive Pro) on older motherboard to gain the full unsupported HD size and then move the HD to a much newer computer which can support the full size (thru Auto detect) but on the Secondary Controller or as Slave to BOOT drive many times the Partitions and DATA look all screwed up because you are not loading the DDO which handles the translation used to support the HD on the older BIOS which could not support the full HD size..
I believe Testdisk wrote a DDO to the drive in order to change the sector size...and the HD was partitoned and formatted before new DDO was added..
Next issue is 2048 is never used for a sector size on a DOS/Win HD.

AirIntake wrote on 2022-02-04, 20:44:

I think HDAT2 has detected the HDD as a CDROM drive! I'm reading through HDAT2's manual though and I'm not sure what to do next.

That is because you allowed Testdisk to write to the HD with 2048 byte sector size which is only used on standard PC's for CDROMS.
Most likely you will need to totally wipe the drive (MBR, boot sector, partition tables, etc) and then start as if the drive was new with no data.
edit: sorry fixed a typo.