FYI this same error occurs when I try to assign 9 virtual network interfaces for a new VM as well. So its not just a limit on drives.

@sidneymussburger
Any conclusion to this? Did you investigate your GPU’s integrated graphics as the primary card In your BIOS?
@r1
Is that a requirement of PCI passthrough? That it can’t be the primary GPU? I started a thread recently trying to debug a passthrough issue.. and finding this info I think it may be related. But my system has no onboard GPU. Can it not run headless? CPU is 1950X by the way. It does seem like dom0 is competing for use even when its supposed to be using the pciback driver. More info-
https://xcp-ng.org/forum/topic/3952/nvidia-gt-710-gpu-passthrough/5