I’m a convinced agnostic, but I’m pretty sure that no “D8B god” exists - haha!
I just started on PCs from that era (ca, 1995-2003… aka BIOS… pre EFI), so I remember that side of things pretty well. For the innards of the console, beyond making sure what should have contact really HAS good contact (and what shouldn’t, doesn’t)… and settings… like the BIOS, yes… I don’t really know more than “common knowledge” about that era of computers, as used in the D8B system.
So, I think I already spilled most of what I know here… but we can dive in, if necessary
Sometimes the BIOS can get a bit stubborn, and even IF the battery still might have enough juice to store the settings - it can be a good idea to remove it and let the mainboard sit with no power and no battery for a while, to let it rest. At least 15 min, I’d say… but if there’s no improvement, I’d let it sit with no battery and disconnected from the wall over night (as silly as that sounds).
If the battery has never been (or not within the last 3-5 years) been replaced, then it would definitely be a good idea to replace the battery while you’re at it, to avoid new problems soon down the road, even IF you could get into the BIOS with the current battery and have it remember the settings… if the battery fully dies after all, soon, youd’ have to repeat this whole procedure.
Then, after hopefully being able to access the BIOS, you’d re-do the BIOS settings. And yes… if the PS/2 mouse connector is on a separate PCI slot plate, then it should be the “new” board.
I think the question to either start out with OS3 and the old hard drive, or OS5.1 and ideally with an externally accessible CF card, is mostly of financial nature.
If you want to be sure you can get the D8B running without investing any money into it at first, then you could just clone OS3 as it is right now (to an image, as a backup), re-install from the floppies (if you have them or can write images), and just try if the mixer is “back” before investing into a CF setup (which isn’t THAT expensive, though).
So, someone cheap like me, would first try if a unit runs “as-is” with minimal interaction and as far as possible, no money upfront, haha. So, new battery (thanks csp), then cloning the “old" IDE drive via a USB adapter as a backup.
Then restoring arjpesen’s clone of a working OS 5.1 installation back to the D8B’s internal IDE drive “for free” (as long as you have the necessary Macrium Reflect cloning software or trial version of it, and finish the project during the trial period).
Also, I think OS5.1 requires at least 64 MB of RAM to run well, though. How much RAM do you have installed (for such a “free” OS 5.1 test)?
Otherwise, if you’d rather do it once with the final hardware, than do a trial-run with the existing hardware and available options, then I’d highly suggest to go with a CF card and OS5.1/hack… then you’re only limited by the FX (and I/O) cards you have installed… not by whatever add-ons the original owner of the desk spent extra money on, in form of add-on FX plug-in licenses, etc’.
If “unlucky,” the original owner might not have bought anything extra, and only get the basics with OS3, with none of the 3rd party plug-ins available then, and after.
With 5.1 and the (easy enough) hack, you’d get all the plug-ins you have FX cards for (ideally MFX and UFX, to get ALL plug-ins). To do the hack, you pretty much just replace the executable file with a hacked version available on this forum (easy search, but I can find the link, if it doesn’t pop up easy enough.
So… again, if cheap like me, I’d be prepared to do the whole exercise twice… once “for free” with existing hardware (clone old drive incl. boot sector as a backup, then restore OS5.1 image to “spinning” IDE drive), and after then troubleshooting the boot-up hang with that “fresh install”, I could still decide if I want to spend money on the CF card/adapter, in case the whole thing turns into a similar saga/fiasco like in csp's referenced case (…a god would have just revived that unit… but it’s still as good as dead, despite of my best efforts to help… ha!)
Anyway… that’s my 2 cents… but what I’m talking about above can also easily turn into many evenings of tinkering and trial and error… so… many of us here have been there, and we’re here to try to help, if getting stuck!
(And all of them really nice, modest people, from all I know… hopefully I’m not underestimating anyone here drastically (“gods”?!?)