How many CF cards have you tried? I've been using XT-IDE rom across several machines (have it on a 286, a 486 and a P166 on two 3c509s and an rtl8139) for many years and there are times when it just wouldn't work with particular CF brands/models.

I tend to stick with SanDisk Ultra and Extreme models these days as they tend to just work without issue.

The adapter plugged in at the moment is the generic "ASHATA SD to 3.5in IDE,SD/SDHC/SDXC/MMC Memory Card to IDE" (amazon), I've tried it with a 2GB, 4 GB and 8 GB SD cards (a mix of Kingston and generic brands), the CF adapter is a "Ximimark 2Pcs Compact Flash CF Card to 3.5" (amazon) and for thats if tried 512 MB and 2 GB cards both genric and "Industrial", and I've also tried a 2GB Disk on Module device (trying to find the Amazon link for that one), that module is currently in a second PC and working just find as a boot device with NT4 installed, but before I moved it to that computer I tried it on this one and it did the same thing, BIOS sees it, I can fdisk and install MS DOS on it, but when I tryto boot from it it just hangs. I haven't tried that one on XTIDE yet but I was hoping there was some diagnostic I could try before I wipe my NT4 install on that one.

Are you trying to boot with just this SD2IDE adapter connected to IDE channel? Maybe the incompatibility is related to some strange behavior of this BIOS which is, by some strange issue, seeking for the second drive. I would suggest to check with 2nd drive connected to IDE cable (either checked while being slave or master). Maybe the result of this check will help to narrow down investigation of the root cause

sndtst wrote on 2023-01-07, 19:23:

The adapter plugged in at the moment is the generic "ASHATA SD to 3.5in IDE,SD/SDHC/SDXC/MMC Memory Card to IDE" (amazon), I've tried it with a 2GB, 4 GB and 8 GB SD cards (a mix of Kingston and generic brands), the CF adapter is a "Ximimark 2Pcs Compact Flash CF Card to 3.5" (amazon) and for thats if tried 512 MB and 2 GB cards both genric and "Industrial", and I've also tried a 2GB Disk on Module device (trying to find the Amazon link for that one), that module is currently in a second PC and working just find as a boot device with NT4 installed, but before I moved it to that computer I tried it on this one and it did the same thing, BIOS sees it, I can fdisk and install MS DOS on it, but when I tryto boot from it it just hangs. I haven't tried that one on XTIDE yet but I was hoping there was some diagnostic I could try before I wipe my NT4 install on that one.

You might be hitting a variant of this issue as your SD2IDE adapter probably uses an FC1307 .

Re: Using a vintage multi-track recorder as a mixer, namely the Roland VS-880EX - might apply to other Roland VS- units

I finally figured it out. I tried removing as many variables as possible, and in the end, it had nothing to do with the XT-IDE or even the various adapters. The partition on the SD cards and on the CF Cards all lacked a Master Boot Record (even when I deleted and recreated the portion tables). In final answer was to run

1fdisk /mbr

Then each disk booted without an issue. Thank you everyone for your ideas, I learned a lot during this project and now I have a slightly more reliable and quieter P166-MMX now 😀