~ This ~
[*]Disconnect 3.5" drive A and just connect the 5.25" drive and see if issue persists
[*]Swap known working drive from other system and test in this one and see if issue persists

DamienC wrote on 2019-12-31, 00:49:

Is there anything else I could check in the meantime to figure out why this is happening?

Not like often but (in forums) I've seen a few old FDD with bad capacitors show up lately.
Capacitors wear-out so that should more or less be expected with anything that old.
Fixing FDD is not my thing. I've never even tried to fix one.
.
Look for caps that are leaking or bloated but not all bad caps fail with visual indicators so that's no where near a 100% reliable.
.

PCBONEZ is correct about possible bad caps, there are about 7 and are very small 6.3 to 33uf(iirc) on the Teac 55GFR and a few handle the stepper and spindle motor. Many old floppy drives have been brought back by cap replacement and a good cleaning. It is odd that a DD 360k cannot be read because it has twice the track width as HD 1.2Mb. Usually it is the other way around where 360's work and 1.2's don't as the drive fails (from my experience). Are you sure the 360's are still good ? Over time the original magnetic recording dissipates on disks as it does on VHS tape and become unreadable.

PCBONEZ wrote on 2019-12-31, 01:22:

~ This ~ [*]Disconnect 3.5" drive A and just connect the 5.25" drive and see if issue persists [*]Swap known working drive from […]
Show full quote

~ This ~
[*]Disconnect 3.5" drive A and just connect the 5.25" drive and see if issue persists
[*]Swap known working drive from other system and test in this one and see if issue persists

DamienC wrote on 2019-12-31, 00:49:

Is there anything else I could check in the meantime to figure out why this is happening?

Not like often but (in forums) I've seen a few old FDD with bad capacitors show up lately.
Capacitors wear-out so that should more or less be expected with anything that old.
Fixing FDD is not my thing. I've never even tried to fix one.
.
Look for caps that are leaking or bloated but not all bad caps fail with visual indicators so that's no where near a 100% reliable.
.

Well... I'm starting to think the drive is just messed up.

I took my other 386 out of the attic and yanked the 5.25" drive. I thought it was the same model, but it's actually an FD-55GFV. I'll refer to this as "good drive" and the FD-55GFR as "old drive." Results:

Good drive from 386 (prior to removal): Drive reads & writes all disks
Good drive in P83: Drive reads & writes all disks
Old drive from P83 installed in 386: Drive reads & writes HD 1.2mb disks, "Sector not found" when trying to read 360k disks
Old drive from P83 restalled in P83: Drive reads & writes HD 1.2mb disks, "Sector not found" when trying to read 360k disks

The drive doesn't appear to have any blown capacitors. It was sitting on a shelf in my non-climate-controlled attic for over a year, and I'm almost positive it was working fine back then. So I'm not quite sure what's wrong with it.

I'm just going to swap to the drive from the 386. Only slightly annoying thing is it has a black bezel and the other one is beige. I was sorta thinking of switching to a black case anyway.

Power it up.
Let it sit with power for about 2 hours.
Then try it.
.

I had a Mitsubishi 5.25" 360kB drive that would not read disks reliably at all. None of the electrolytics on the board had any visible bloating or leakage, yet when I removed them I found some with almost no capacitance remaining. After a recap the drive works fine.

Yes.
Only -SOME- bad wet-lytic caps bloat or leak.
All of them eventually wear out.

Aluminum Electrolytics are (used to be) called "Self Healing" caps.
With a DC bias applied the oxide layer will rebuild itself and repair any damage to the layer.

When caps sit with no DC voltage applied the layer gets thin by slowly dissolving back into the electrolyte.
There is a point where enough dissolves that the cap will not work right.
If you then put voltage on it a process like electroplating occurs that rebuilds the layer. - Then it works again.
- All of that only works until the electrolyte wears out from age, oxidation and/or contamination.

This is why I said to put power to it for a while then check again.

This is also why you should power-up any stored boards (cards & mobos & FDD too apparently) with wet-lytics for at least 1 hr a year.
30 minutes is probably enough for 1 year, just failing-safe by saying 1 hr.
.

Bloating and leaking tends to come from overheating. Often, internally-generated heat, which is common with caps that are applied across power rails. (Leaking can also occur because of material failure at the bottom rubber seal.)

Now, electrolytic caps are also used for signal conditioning -- like DC-blocking. In this case, they'll be exposed to very little current, and so will not heat up enough to expand and swell. I just recovered a 720KB Teac 3.5" drive by replacing its caps. They looked perfect, but one measured 0uF. So, if you can, give it a shot -- nothing to lose.