ASRock A785GMH/128M fails to find devices when in RAID mode and disks are present

[Xen]

Just want to report that I have a motherboard (A785GMH/128M) that will 
fail to boot the moment a RAID disk is present.

Grub will just not see any devices. It will fail with a message that 
(hd0) could not be found.

"ls" then yields nothing, an empty line as output.

This is also true when booting from USB stick. It will fail to boot this 
stick, even though all required partitions etc. are on the stick itself.

An alternative motherboard with the same chipset that I have lying 
around will boot without issue although I have not configured an array 
on it yet; but this motherboard simply allows "Single Disks" to pass 
through and neither the usb-stick-grub nor the hard-disk-grub have a 
problem with it. So I have yet to test what happens when I take out the 
disk that is currently running this system and put it in that other 
system so that the RAID controller will see a "configured "SPARE DISK"" 
or something.

Motherboard is ASRock A785GMH/128M, socket AM2+, SB700, memtest86 runs 
fine (from an SD-card when the disks are present) and the Kubuntu DVD 
also boots as normal (have not allowed it to run to see if it will 
recognise the "array").

So this is just AMD firmware RAID, using the Promise "pdc" driver. I 
know that on a different AMD motherboard (SB710, 760G) I could not get 
Linux to boot (fully) but it was clear Grub didn't have any problems 
back then. The installed Grub on both test disks (usb + harddisk) is the 
Ubuntu version that is over a year old. I mean that it is at least as 
old as May 2015 as I mentioned under my developer nick on the devel 
list. So I haven't tried an actual new version yet based on current Git.

Is anything like this expectable or known?