Most likely a short on the HDD.

Alternatively but not as likely an aged PSU that can't handle the HDD's spinup power draw.
But I would really suspect a defective HDD to be more likely.

The HDD was working fine with no bad sectors previously. Issue started when I removed the existing sound card and installed the recently purchased one. I was suspecting a loose cable. Tried with another molex connector from the PSU as well as from a molex splitter, but the same result. Quite surprised as to how the hard disk is causing the system to shut down in a few seconds after starting.

What happens when you connect your hard disk just to the molex and not to the controller?

Hope you are referring to only connecting the molex and not the IDE cable. Tried that and the result is the same. PC immediately shuts down after starting.

stealthjoe wrote on 2021-10-21, 15:17:

The HDD was working fine with no bad sectors previously. Issue started when I removed the existing sound card and installed the recently purchased one. I was suspecting a loose cable. Tried with another molex connector from the PSU as well as from a molex splitter, but the same result. Quite surprised as to how the hard disk is causing the system to shut down in a few seconds after starting.

This can be completely unrelated to opening the PC and swapping parts. When spinning the disks up the motor controller has to push much more amps than once the disk is spun up. That plus with time the bearings get worse so it'll have to push even more to spin the disks up.
If the motor controller fails it will most likely happen when powering the system on or waking the drive from sleep.

That's why my money is on the motor controller having failed short from what you describe. Best guess from a remote POV.

Doornkaat wrote on 2021-10-21, 16:09:

That plus with time the bearings get worse so it'll have to push even more to spin the disks up.
If the motor controller fails it will most likely happen when powering the system on or waking the drive from sleep.

Combine this with high ripple on the PSU and people will wonder why HDDs die in series.
I've seen up to 3V ripple on 12V and from what I have read, some motor controllers don't take ripple well.

Maybe we get some information about the PSU and HDD models in question?

Motors when they spin up will draw more current if they are aged than a brand new motor will. This will lead to a larger voltage drop in time as drives age. I never really bothered running more than one hard drive in my computers, but I noted recently how servers and scsi drives and controllers can stagger spinning up the motor to prevent too much current draw, instead of powering up the motors immediately on receiving power. Anyway I suspect old drives will not play nice with old power supplies.

This is why having test bed and spare parts to test with is nice. You can check this drive against a healthy supply and see the voltage & current draw. This allows you to identify if it is the power supply or hard drive. But they both could be on their last legs.

I have seen this same exact issue win the past when a drive died. Acted like something on the controller board or the motor itself had a direct short to ground.

stealthjoe wrote on 2021-10-21, 15:17:

Quite surprised as to how the hard disk is causing the system to shut down in a few seconds after starting.

Most power supplies have an emergency shutdown procedure in case the output voltages are not OK, to protect the computer from damage. As your system starts kind-of OK for a couple of seconds, it seems like the +5V output is good enough. The hard drive is also connected to +12V, and likely draws more power on the 12V line than your supply is able to deliver, triggering the emergency shutdown.

As other people already said, the root cause might either be the power supply that can't deliver the required current to spin up the drive anymore because it got weak, or it might be the drive that draws way to much current when it tries to spin up. You might want to try the drive on another power supply to make sure. Best to use a 180W-300W supply so over current protection kicks in earlier than on an 850W supply

Update: I tested with another new IDE HDD (500Gb) and got the same result. The PC switches off immediately on hearing the disk spin up sound. So thinking if this could be due to the PSU.

BTW my PSU is a chinese make - Foxin FPS 500 (500w). The existing HDD is a Seagate ST3160215A.

This is impressing... that PSU boasts 25A on 12V.

I guess I am not the only who hopes that you'll post a photo of the insides 😀

BTW... this name sounds so similar like "Toxin"... Scary!

retardware wrote on 2021-10-22, 02:38:

I guess I am not the only who hopes that you'll post a photo of the insides 😀

There's a thread for that: PSU showcase: The worst of the bad 😀

retardware wrote on 2021-10-22, 02:38:

This is impressing... that PSU boasts 25A on 12V.

I guess I am not the only who hopes that you'll post a photo of the insides 😀

BTW... this name sounds so similar like "Toxin"... Scary!

Thanks for the article. My PSU is exactly the same as in the picture for 500W (may be the sticker graphics alone is a bit different). Didn't realize this PSU was a dud. Now it brings me to the question - how many min amps is required by the +12V rail to boot a PC (spec in my signature). Sorry I am bit noob at this. Thanks.

stealthjoe wrote on 2021-10-22, 03:33:

Now it brings me to the question - how many min amps is required by the +12V rail to boot a PC (spec in my signature).

Well, the ATX standard did not fullfill the power draw needs of a Pentium IV processor.
So they have extended the standard by adding an extra +12 V cable that should be wired near the CPU on the motherboard. This has been called ATX12V.

Update: I purchased a better quality PSU from Amazon by the name Ant Esports (not Antec despite resembling significantly) and attached it to the board.
Next, the PC booted to the boot selection menu (finally!!!). So it seems like this was a case of nuked PSU indeed. I re-checked the hard disk through Speed Fan and noted all SMART parameters to be normal. Thank you all for your help!!

Below are few pics of the scanty interior based on popular demand (pun intended!):

Very nice to see that the computer works again 😀

Actually, people were most curious about the insides of the old PSU... we all love bulged and leaking capacitors, browned circuit boards and burnt components 😀

Regarding the photos of the computer, you might want to verify that the case fan is mounted that way that it blows into the case and not outwards.
Often these are mounted in a way that they blow outwards, and then the PSU gets starved from cooling by the competing fans, which is particularly bad in hot climates.
If the case fan(s) are mounted so that they blow air into the case, then they support cooling the PSU, which only can be beneficial.

retardware wrote on 2021-10-22, 20:23:

Very nice to see that the computer works again 😀

Actually, people were most curious about the insides of the old PSU... we all love bulged and leaking capacitors, browned circuit boards and burnt components 😀

Oh ok. Coming up:

Looks like some kind of dried electrolyte. But caps look good. The white stuff beside the right capacitor seems like a silver fish home 😀

retardware wrote on 2021-10-22, 20:23:

Regarding the photos of the computer, you might want to verify that the case fan is mounted that way that it blows into the case and not outwards.
Often these are mounted in a way that they blow outwards, and then the PSU gets starved from cooling by the competing fans, which is particularly bad in hot climates.
If the case fan(s) are mounted so that they blow air into the case, then they support cooling the PSU, which only can be beneficial.

This is interesting. Was thinking that the rear fan was supposed to exhaust the hot air from inside. There is also a front mounted intake fan on the case. Would this in any way be beneficial for the PSU?