Not sure how to say it any more clearly. It's a bug because the Proxmox installer does not honor the BIOS setting that has disabled UEFI on the machine. Proxmox creates a GPT disk label type that only can be used on a UEFI-enabled machine. Thus, it will not boot from power on. And, no, "all known BIOS can deal with that." Intel BIOS cannot, and it makes up a lion's share of the server market. Proxmox boot fails on any server with UEFI capability that does not have a UEFI-compatible drive. RedHat, CentOS, Scientific Linux, and Ubuntu all handle this properly. I suspect Debian does as well since Ubuntu is a derivative. Proxmox does not. Simple as that.