On the AS ROCK H81 PRO BTC you can add a maximum of 7 "Add in" GPUS and here is why.
I've seen this question asked quite a few times across various places. Why can't I add an eighth gpu to the H81 even though I have empty slots?
Windows or Linux, it doesn't matter, has a maximum of 8 GPU's regardless of the combination of physical cards. Four 5970's equals 8 GPU's, two 5970 and four 5870's also equals 8 GPU's....I think you get the idea.
Q. Then how come we can only add seven GPU's to the H81 if eight is the limit?
A. The onboard GPU is loaded as GPU 0 in the bios and takes up a "slot" regardless of what you do in the bios. Actually the on board gpu cannot be "disabled", you can only choose which is the primary display device, PCIE or ONBOARD.
Using two 5970 and three 5870 will max out all expansion even though there is an empty slot on the board. You can see why below in the final configuration as loaded into bios using the above cards.
Intel HD Onboard - GPU 0
5970 master - GPU 1
5970 slave - GPU 2
5970 master -GPU 3
5970 slave -GPU 4
5870 -GPU 5
5870 -GPU 6
5870 -GPU 7
As you can see, the above configures 8 gpus including the on board gpu using 5 out the 6 pcie slots.
~BCX~
The on board gpu automatically take one IRQ resource on the board, but there should be a way to trick the registry to add the 8th external gpu.
