Hi, try SDFormatter to clean you SD card.
Then put it back in that PC and make sure the BIOS is recognizing the card correctly (enable or disable LBA).
If you can, disable thing like "block tranfer" or "32bit transfer". Set PIO modes to 0-3 (PIO mode 4 is kwnon to be buggy).
Then try to install DOS 7 .1 on this machine first (can't believe I'm saying this!), because DOS 5/6 doesn't support LBA (it doesn't have to, but LBA can circumvent issues with very large drives).
Then try to reboot from this machine with the sd card installed.

SDFormatter
https://www.sdcard.org/downloads/formatter_4/

Jo22 wrote:

Hi, try SDFormatter to clean you SD card. Then put it back in that PC and make sure the BIOS is recognizing the card correctly ( […]
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Hi, try SDFormatter to clean you SD card.
Then put it back in that PC and make sure the BIOS is recognizing the card correctly (enable or disable LBA).
If you can, disable thing like "block tranfer" or "32bit transfer". Set PIO modes to 0-3 (PIO mode 4 is kwnon to be buggy).
Then try to install DOS 7 .1 on this machine first (can't believe I'm saying this!), because DOS 5/6 doesn't support LBA (it doesn't have to, but LBA can circumvent issues with very large drives).
Then try to reboot from this machine with the sd card installed.

SDFormatter
https://www.sdcard.org/downloads/formatter_4/

I'm beginning to think the adapter is at fault. I got booting from the "hard drive" working again, but after a few reboots I got "invalid system disk" - put the card into a card reader, windows claimed it wasn't formatted, and a partition program agreed with this! They're great things for quickly transferring data without burning cd's or flash drives, OR usb-ide adapters, but just disconnecting an adapter and reconnecting it with the power off seems to be a risk to corrupting the card. Is there any reason an SD card would suddenly show up as unformatted even when it's being read in a device other than the one that can't see it as formatted?

How much is the size of the SD card ? Everything beyong 2GB is usually SDHC or SDXC.
Older hardware like card readers. flash cards or cameras may not work with them, because newer SD types need special cntroller support.
And other old devices seem to work with 4GB SDHC cards at first glance, but may suffer data corruption at some point.

Jo22 wrote:

How much is the size of the SD card ? Everything beyong 2GB is usually SDHC or SDXC.
Older hardware like card readers. flash cards or cameras may not work with them, because newer SD types need special cntroller support.
And other old devices seem to work with 4GB SDHC cards at first glance, but may suffer data corruption at some point.

512mb. Strangely a socket 7 motherboard will recognize a 32gb card in it, but I dare not try to actually put anything on it that I want to last on there.

It definitely won't work with SDXC cards, however would partitioning it as 32 or under be a workaround?

I know that some Socket 7 boards do have a 8GB limit, but 512MB should be fine. That capacity is within the 2GB SD card limit
and should work as well with systems using CHS addressing (never seen a Socket7 system without LBA).
Maybe it's just a BIOS bug ? Yes, you can try to create parition with a different size. A smaller partition shouldn't hut.
But never use the low-level tool in the BIOS. it causes strange things and could permanently damage the card.