Good news is if you're up to this point, it's a software issue. BIOSes with the 8.4GB limit can handle IDE up to 16384 cylinders, 16 heads, and 63 sectors. But the interface between the BIOS and DOS only allows 1024 cylinders. 1024/16/63 is 504MB. Luckily enough, that BIOS to DOS interface can handle up to 255 heads. So BIOSes in that era (1993-1994) gained the ability to do LBA-assisted translation, in which DOS is presented with fewer cylinders but more heads, and this illusion is reversed when talking directly to the drive.

Long story short, you need to look for an LBA option in your BIOS and enable it (as opposed to CHS or NORMAL), repartition, reformat.

Your problem isn’t about the adapter, you just don’t have LBA enabled in BIOS which limits the usable size to 504MB in normal mode. Set LBA on and you get the full 1GB available (your BIOS definitely has LBA if it supports up to 8.4GB).

Thank you both!

Combined with flashing the master boot record (the card *was* from a camera so likely that had something to do with it), switching to LBA fixed it!

UPDATE:

We got the 1GB working just fine. DOS boots.

We tried to replicate the process on an 8GB card we got. Everything is identical. But DOS does not want to boot on the 8GB card. It recognizes it, we even can get it to boot on a virtual machine with the CF card. Gparted and Windows both read and recognize it, and we've enabled the boot flag (to match the 1GB). Any advice on the reasons this may not be working?

Capybara wrote on 2025-08-23, 20:15:

UPDATE:

We got the 1GB working just fine. DOS boots.

We tried to replicate the process on an 8GB card we got. Everything is identical. But DOS does not want to boot on the 8GB card. It recognizes it, we even can get it to boot on a virtual machine with the CF card. Gparted and Windows both read and recognize it, and we've enabled the boot flag (to match the 1GB). Any advice on the reasons this may not be working?

DOS parition size limit is 2GB, so it won’t work and you need to paritition it to four separate parititions. I wouldn’t bother, though, 1GB is plenty for such system if you just don’t hoard stuff on the card. Install what you actually use and you won’t run out of space. For a system like that couple of hundred megs was probably the average at best, often less, especially when people upgraded their older machines and kept the hdd.

Capybara wrote on 2025-08-23, 20:15:

UPDATE:

We got the 1GB working just fine. DOS boots.

We tried to replicate the process on an 8GB card we got. Everything is identical. But DOS does not want to boot on the 8GB card. It recognizes it, we even can get it to boot on a virtual machine with the CF card. Gparted and Windows both read and recognize it, and we've enabled the boot flag (to match the 1GB). Any advice on the reasons this may not be working?

It's best to partition and format the card in the machine it will ultimately be used on, even if you then take it out and put it back into another machine to bulk-load software.

It's most likely: a mismatch in the translated geometry in the 486 vs. the new machine that formatted it, or you used Win95B+ aka "DOS 7.1+" fdisk or another partitioning utility that partitioned it assumed it was being used in such a system and used partition type 0C instead of 06, or it's FAT32, or many other possible reasons.

If you definitely want to use a modern machine to partition, format, install DOS, you could use PCem or 86box to build your raw disk image, so that it's a "period correct" BIOS and partitioning software involved.

jakethompson1 wrote on 2025-08-23, 20:44:

If you definitely want to use a modern machine to partition, format, install DOS, you could use PCem or 86box to build your raw disk image, so that it's a "period correct" BIOS and partitioning software involved.

It's been a hot minute since he's used 86box. How does he have 86box mount the compactflash in terms of partitioning, formatting and installing DOS?

Capybara wrote on 2025-08-23, 23:27:

jakethompson1 wrote on 2025-08-23, 20:44:

If you definitely want to use a modern machine to partition, format, install DOS, you could use PCem or 86box to build your raw disk image, so that it's a "period correct" BIOS and partitioning software involved.

It's been a hot minute since he's used 86box. How does he have 86box mount the compactflash in terms of partitioning, formatting and installing DOS?

I'm on Linux so I have removable drives as /dev nodes that I can attach to qemu or other such software as a disk. Failing that, you could make a disk image file exactly the size of the drive, set it up exactly as you want it, then dd the whole thing to the CF.

It would still be better to partition it in the machine it's going in.

Thank you. We'll try that and I'll let you know how it goes.

We can't get fdisk in the 486 to work properly with it, so that's why we're trying it externally.