I'm assuming from you mention of the switch "on the PSU", and "fireworks"
that this is an older-style AT supply (power switch on mains - cabled directly
to the supply), and not an ATX (power switch on mainboard jumpers)

In this case, assuming the PSU casing is properly grounded (3-prong power
cord plugged into grounded outlet) -shorting mains to ground shouldn't damage
anything with the system - for all intentional the short would have been
effectively external to the system.

Most likely there has been a fault for another reason which happened about the
same time - possibly induced current spike or a gound loop - both very unlikely.

Does the floppy seek/home head during POST? Try moving the head all the way
out (inner tracks) and see if it homes to outmost track during POST.
This will tell you if the FDC selection logic and step signals are working
and addressing the drive.

A good tool for testing low-level floppy operation is ImageDisk - this will
let you perform basic seeks, see if the drive is reporting index pulses,
track-zero detect etc.

I suggest trying "Clean head" - this will seek back and forth as long as the
drive is seeing index pulses and not require any data from the drive.

Your BIOS screenshot shows both 5.25" and 3.5" drives - are both not working?
Does the 5.25" drive have a termintion jumper resistor pack?

DaveDDS wrote on 2025-07-26, 18:32:

I'm assuming from you mention of the switch "on the PSU", and "fireworks" that this is an older-style AT supply (power switch on […]
Show full quote

I'm assuming from you mention of the switch "on the PSU", and "fireworks"
that this is an older-style AT supply (power switch on mains - cabled directly
to the supply), and not an ATX (power switch on mainboard jumpers)

In this case, assuming the PSU casing is properly grounded (3-prong power
cord plugged into grounded outlet) -shorting mains to ground shouldn't damage
anything with the system - for all intentional the short would have been
effectively external to the system.

Yup, you are correct! I am pretty sure ground was in place then...

DaveDDS wrote on 2025-07-26, 18:32:

Most likely there has been a fault for another reason which happened about the same time - possibly induced current spike or a g […]
Show full quote

Most likely there has been a fault for another reason which happened about the
same time - possibly induced current spike or a gound loop - both very unlikely.

Does the floppy seek/home head during POST? Try moving the head all the way
out (inner tracks) and see if it homes to outmost track during POST.
This will tell you if the FDC selection logic and step signals are working
and addressing the drive.

A good tool for testing low-level floppy operation is ImageDisk - this will
let you perform basic seeks, see if the drive is reporting index pulses,
track-zero detect etc.

I suggest trying "Clean head" - this will seek back and forth as long as the
drive is seeing index pulses and not require any data from the drive.

Your BIOS screenshot shows both 5.25" and 3.5" drives - are both not working?
Does the 5.25" drive have a termintion jumper resistor pack?

Yes, I can hear both drives are seeking during POST. Also, they are both spinning when trying to access the drives (and giving the error message).

The 1.44mb doesn't have any jumpers, while the 1.2mb only has what to appear to be for device id selection (see attached images).

Linoleum wrote on 2025-07-26, 19:09:

The 1.44mb doesn't have any jumpers, while the 1.2mb only has what to appear to be for device id selection (see attached images).

I wasn't referring to "jumpers" .. 5.25" drives often have what looks like an IC socket where a 150ohm
Terminarion pack goes - this is only for the drive at the end of the cable. 3.5" drives (and some newer 5.25"
drives) doen't have this and have partial termination built in. As the drive was working before and
you've not changed it, this won't be the problem, it's just part of what I look for when testing out a
floppy drive.

If you have a scope, you could look at the data signals and see if the drive is sending good signals (If it works
on other systems, it most likely is - but always worth checking to see if the data line is excessively pulled
high or low on the mainboard side.

Well, I just tested one of the drives (the 1.2MB) on another system, and it worked right away—so the issue doesn't seem to be with the drives themselves.

Another possibility is that my original I/O controller (Kentech Computer IDE PLUS-V2) has gone bad, and I may not be configuring the replacement correctly. See the picture: most settings are left at their default (marked with an asterisk), except for the ones I changed, which are indicated by arrows.