What brand of memory cards are you using?
They are not all created equal.
I've heard of trouble using Kingston brand cards, while I use Sandisk and Transcend and they seem to work ok.

Generally the IDE to CF adaptors work very well and are not really high tech, so your problem does not need to be with the adaptor at all. When FDISK worked fine good chances the hardware itself is okay or that should already fail. You did not mention how large your CF is. Also you did not mention which version DOS you are using. If Fdisk can kill all partitions then unix can also do not much more for you i fear. My advise would be to try installing like win95 as a test, that should be quite fast with a CF and if that works it is more a software config problem. If win 95 also wont install or run properly then i can only suggest to try a different CF card, a general recommendation would be like a 2 GB card or you may need to partition it. If win95 works then it is probably a problem of your partitioning strategy/DOS version, keep in mind that older DOS versions can not access a bootloader that is beyond the 2 GB limit.

Yep, I'm using Dos 6.22 from floppy. I could try either Win98 or freedos instead too.

The CF card was 4GB Lexar, although the motherboard utterly refuses to see it at all, even when formatted with a 1GB FAT16 partition on my main PC first.

The SD cards are generic 2GB cards from a manufacturer that nobody has heard of (Optima). Seems to work fine when used "as it was originally intended!"

Maybe a better SD card would help, although I'm surprised I got this far as even the "Starting MS-DOS" at all.

The sd cards adapter might be limited to only smaller sd cards. 512Mb or 1Gb. You can try to find out about this.

hmmm, im not having good memery, but i think DOS 6.22 should support "large disks". Generally speaking as you say "the motherboard refuses to see it" is indeed a sign of hardware compatibility.

Reminded me a bit of my fight with my IBM PC-110 from 1995 which was bitching a lot on large CF cards. Although the internal CF reader can "see" the card and starts to boot from it i usually only get funny DOS messages like "No valid system". What is funny is that you can see it starts loading the bootloader as a CF formatted in my german windows 98 comes up with the same message - but in german 🤣. Although i tried a dozen of large cards it was simply not possible to use anything larger than 2 GB. At least for the PC-110 i am sure this is a BIOS related problem. With "most" CF cards of 2 GB or below it worked fine, but with some "high speed" cards. Some super fast cards did not work while generic no name cards worked fine. Ironically for the PC-110 slower meant better as the fastest ones simply didnt work.

On my SS7 I get CF corruption on the primary IDE port. So I tried the secondary and that one worked fine.

My CF cards also work on my 486 (OEM Acer). But I don't have anything much older to test.

The CF card has to show up in BIOS, even if there is no partition on it. You might have to go into the HDD section and do a scan or set the parameters manually.

OK i have a question about this.

Is it worth to use SMARTDRV when using SD Cards of CF cards as hard drives? Because 28K of memory is worth saving...

IMO no, SMARTDRV isn't worth it for CF cards.

I'm actually using Industrial Grade SD Cards which use more reliable SLC instead of more common MLC. My 1GB card scores something like 0.60ms average access time on speedsys. Reads are very fast too, writes are not very impressive but i would say at first hand SMARTDRV is not needed too. It might be worth doing some tests.

In the mean time maybe other users can report their experiences about the subject.