I've got an adapter that only goes up to 40GB because it's 20 yrs old.

ATA (aka IDE or EIDE) was all they had before SATA so these adapters were all over the place.

WTF is a Pi for?

There's small little chips that do exactly this interface, still probably plenty available surplus for cheap since ATA is not popular any more.

Any ATA HDD that works on a mainstream USB adapter will handle CHS for DOS usage just fine, or LBA. Pretty much automatically. This was already smoothed out before about 1994 or so.

DOS of the '90's could handle LBA anyway, I like the MS-DOS from Win98SE which was not available separately by then but after W98 is installed to a FAT32 partition, the key DOS OS files in W98 will be available to copy to a fresh fat32 volume. You pretty much copy the same (newer) DOS files from W98 to your DOS root that it would have after installing DOS 6.22

Here's a USB adapter now:

https://www.amazon.com/SATA-Adapter-Converter-Cable-Drive/dp...

Not only that but Windows 10 will still install and run on drives with these old connectors just fine in BIOS mode as expected but also partitioned & formatted with the latest GPT HDD layout, GPT would boot with UEFI rather than BIOS for most mainstream configurations.

But also if you do it right, GPT will boot a plain MBR HDD if the UEFI can find a suitable FAT32 BOOT folder.

UEFI + GPT alone is just too lame to run DOS on bare metal any more. Not without BIOS mode or CSM enabled. GPT is still a show-stopper though, DOS needs MBR drive layout :(

OTOH MS-DOS will still install & run on any PC or drive, even NVMe, as long as there is a CSM or legacy BIOS mode and you are not stuck with a crippled mother board having only UEFI. GPT-only means no DOS for you. Not without some kludge like a virtual machine.

The purpose of UEFI + GPT was to make it so you couldn't run DOS, Win3.x, W9x, Wxp, Vista, W7, and Linux ever again on bare metal.

Not without CSM. The key UEFI module.