It could be that there's something left in the boot sector that neither FORMAT nor FDISK do remove.

Some part of a DDO, a boot manager, a backup tool, a cmos setup utility etc.

If that's the case, S0Kill may help. It cleans track 0.
It's usually safe, I think, as it uses normal BIOS calls.
Re: IDE to Compact Flash as MS-DOS boot drive.

Edit: The utility is in German, too, btw! 😁
But it's really not difficult to use.
It essentially asks for a drive letter and if standard parameters should be used (the utility can erase whole HDD if being told).

Kouwes wrote on 2024-02-28, 17:12:

I have this 286-16 Headland HT12 PC with 4MB and a 80287 NPU, works well. Or should I say, worked well. It came with a german ve […]
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I have this 286-16 Headland HT12 PC with 4MB and a 80287 NPU, works well.
Or should I say, worked well.
It came with a german version of DOS 5.0 installed so I took my english 6.22 disks but nope, refused to install over 5.0.
Then I used the 6.22 upgrade version, which worked ok until a message popped up saying I need to insert an empty disk labeled UNINSTALL #1.
Well, I couldn’t be bothered with that so I just formatted the C:\ drive (42MB Conner). After that, I was able to install DOS6.22 but after reboot the same message popped up: insert…uninstall bla bla.
Ok so I used Fdisk, deleted & recreated the partition on C:\ and obviously formatted the drive.
Guess what happened after yet another DOS6.22 install…..
Then I thought, maybe I should try and disconnect the 2nd harddrive, a Conner 250MB.
That gave me a HDD controller failure so I reconnected the drive.
And that was the end of the PC: now it won’t post anymore and the screen stays black.
PSU fan is running btw.
I just can’t think of any reason why it died so fast.

Does it post when you remove the harddisk controller (and so all harddisks and floppy drives)?

@CoffeeOne: haven’t tried that yet, but assuming it posts. That would imply I somehow managed to kill the controller card? 😳

@Jo22: that’s an interesting utility. Never heard of it but I sure would like to try it!

Make sure you didn't plug the 250 back in upside down.

The previous pitching a fit while 250 disconnected might be because older drives, the connor still on there, need different jumper configs between master and single drive. Typical connor settings shown, post model number if you need exact...

1   +---------------+----------+-----------+----------+
2   | Jumper        | 1 Drive  | 2 Drive   | 2 Drive  |
3   | Configuration |          | Master    | Slave    |
4   +---------------+----------+-----------+----------+
5   | -ACT          | CLOSED   | Note 1    | Note 1   |
6   +---------------+----------+-----------+----------+
7   | -C/D          | CLOSED   | CLOSED    | OPEN     |
8   +---------------+----------+-----------+----------+
9   | -HSP          | OPEN     | OPEN      | Note 2   |
10   +---------------+----------+-----------+----------+
11   | -DSP          | OPEN     | CLOSED    | OPEN     |
12   +---------------+----------+-----------+----------+

Took a while, because my PicoGus arrived 😅, but I tried removing the controller card. Well, then it will post!
Both HDDs are connected the right way. You can’t mess that up because there’s just no room to turn the cable 180°.
Reseating the controller card returned me to a no posting system…oh well.
One other thing I noticed. I’m not 100% sure, but I think the PSU fan is now making way more noise than before. That is, making much more rpms?
Whatever, I don’t get it.
I only disconnected a harddrive. Then reconnected it and BAM, system won’t even post anymore??

Kouwes wrote on 2024-03-01, 15:59:

Both HDDs are connected the right way. You can’t mess that up because there’s just no room to turn the cable 180°.
(...)
I only disconnected a harddrive. Then reconnected it and BAM, system won’t even post anymore??

It's up to you but I would reconsider that first statement with the symptoms you get. Perhaps try a different cable?

Yah, a did you do what you think you did, or did you miss a row of pins, or did you strain the connectors on a weak cable so the "sharkbite" crimp terminals lost contact or cut into fragile adjoining wires, or how is Murphy, Eris or Loki screwing with you today?

Kouwes wrote on 2024-03-01, 15:59:

Took a while, because my PicoGus arrived 😅, but I tried removing the controller card. Well, then it will post! Both HDDs are con […]
Show full quote

Took a while, because my PicoGus arrived 😅, but I tried removing the controller card. Well, then it will post!
Both HDDs are connected the right way. You can’t mess that up because there’s just no room to turn the cable 180°.
Reseating the controller card returned me to a no posting system…oh well.
One other thing I noticed. I’m not 100% sure, but I think the PSU fan is now making way more noise than before. That is, making much more rpms?
Whatever, I don’t get it.
I only disconnected a harddrive. Then reconnected it and BAM, system won’t even post anymore??

OK, so when it does not post with the controller card inside (also disconnected any drives connected to the controller card to be sure?)
then it is safe to assume that the controller card is toasted.