Hi, same problem here with Proxmox 5 but without a disk fail. I have simple (and clean) reboot the host (with 3 hdd in raidz1) and fail. Grub Rescue + Unable to find the device id...
I have made an ls and I see all the three disk (3x6tb). If I made an insmod zfs + ls (hd0,gpt2) it said unknown...