Do you have any speed enhancements enabled in the BIOS or is the drive just running in PIO-0 mode?

edit: 🤣, enchantments

Jorpho:
I don't think so, it's a pretty standard setup, but I'll look into it, maybe run with a minimal configuration and seen if anything happens

jwt27:
What kind of speed enhancements are you talking about? The bios is pretty much set up for maximum performance and I'm not sure if any setting might affect this.
The drive is running in PIO-4 as far as I can tell (UDMA in Windows). Besides, reading files from the disc which has the ISO on it, works fine. It's just the mounted image that's slow, so I don't think it's the HDD. I've also tried different disks, including a PCI raid controller that's in the system, but the effect is always the same, so I'm thinking it's a software problem.

With enhancements I meant PIO > 0, multi-word DMA and UDMA modes. What happens when you mount the ISO in Windows using Daemon Tools?

And to be sure it's not caused by fragmentation, you could try Defraggler or some other defrag tool that shows fragmentation of individual files.

What does SHSUCDHD mean with "Not enough room to save the SDA ?"

i have plenty of HDD Space and RAM 😳

Try the /V switch and compare with "mem" in DOS.

/V shows nothing of intreset.

I have 1038Mbytes Free RAM and 594Kb low Mem.