Hi Joe,
Did you try to reseat the CF card, already (while the D8B is off)? Maybe it's just not making contact as it should? If what I'm saying below doesn't work, you might also want to check/reseat the cable connections of the mud duck system to the mainboard, etc..
If that doesn't work... I don't really know what's pre-installed on the mud duck system. Is it the 5.1 OS with the "old" crack, and the plug-ins pre-authorized? I think the mud duck solution was around before the new crack was around, right? Anyway... shouldn't make a difference. In the end, you'll just need "any" working/authorized version of MackieOS 5.1 on a (new) flash card, so you could use either crack to make that happen.
Do you have a card reader for your CF card on your PC (or Mac... but I don't think you can easily make a working clone from the Mac side)? And maybe even a spare CF card?
If so, I'd just play the CF card image/clone onto a 2nd CF card (via your computer), put that in your D8B, and see if this solves the issue. That's kinda the whole point of having the card easily accessible from the card slot in the back.
I looked for instructions right now, and this thread has two good links in the top post, so maybe start there:
http://www.sonido-7.com/d8b/tpis.html#Post21If you DON'T have a spare CF card, you could also backup what's on your CF card right now (e.g. via the Macrium trial on PC), then format your existing CF card, and clone a FRESH image (not the one from your existing CF card... that one probably has an issue or damaged sectors) to your existing CF card again.
But IMO, it would be better to keep your original CF card as a sort of backup, and just mess with it again, after the D8B works with a different CF card... but that's just how I'd personally go about it.
From my experience, (NAND based) CF card sector corruption (if that's the root cause, not sure), is not quite as critical as when sectors on spinning harddrives go bad. If a sector goes bad, it corrupts the data that was stored there... however, when re-formatting, those sectors usually just get marked as bad, while the rest of the CF card often times still works fine... without the corruption necessarily spreading to other sectors of the drive (like it typically does on spinning drives... there, once a sector goes bad, it's kind of a countdown until others will, too). For example, I have a NAND based memory card in my old Wii (hacked to accept Japanese and US games), that reported bad sectors, and it's been working for many years after formatting and getting the bad sectors marked. So, after formatting & cloning or reinstalling the MackieOS back to the same CF card, you have a good chance of it working just fine again. But that's only if the cause was indeed sector corruption on your CF card.
It's also possible that something else went wrong with the Mud Duck system, or hardware on your D8B rack... but IMO, trying another CF card, or re-installing the OS to the existing one, is the easiest of all options to get started with, and wouldn't even make it necessary to take the rack unit apart.