Opterion server board, was working happily, then I tried adding a PCI-e USB 3.0 card. Would not reboot after poweron, even if the card is removed.
Goes through bios, hits grub, gets to "Loading kernel", hangs (i.e., ctrl-alt-del does not reset, requries reset or poweroff).

Reset the bios to failsafe defaults, purged nvram, no help: "Loading kernel", newline, hung.
Pulled battery did hardware reset, went into bios, loaded failsafe defaults, ditto.

I've used a unetbootin image of gentoo stage3 to boot his machine before for initial installation.
Tried that, same result: Loading Kernel, newline, hangs.

In case of hardware issue I've stripped the disk controller and drives & riser cards leaving nothing but a bare motherboard + cpu's + memory.
Same result.

Tried Knoppix via unetbootin of KNOPPIX_V7.4.2DVD-2014-09-28-EN.iso.

This is where things get wierd.

Choosing "debug64" from the main menu gives me:
loading /boot/ ... /linux64 OK
loading /boot/ ... /mininit.gz OK
early console setup
early console decompress_kernel
Bootking the Kernel
<hangs>

Choosing "knoppix64" gives slightly different:
Loading ... linux64
Loading ... minirt.gz
early console decompress
decompressing linux Parse ELF done
Booting linux kernel
<hangs>

Choosing "Default" works with:
Loading /ubnkern
Booting linux kernel
<things happen and the system ends up in window manager>

Q: Does anyone know where the documentation for ubnkern, or the differences between choosing knoppix64 or debug64 and Default are described?
Q: Anyone else with an H8DMU+ that has applied any workarounds they can share?

thanks