Update
I got this working. I re-installed to 2 x drives connected to the onboard controller and re-did the procedure to create the bootable USB drive. I then detached the drives and the system had an entry for proxmox in the boot menu this time. I reattached the original drives and had to do a zpool import rpool. Just to be sure I went over the usb creation again just to be sure its UUID got added to /etc/kernel/proxmox-boot-uuids
Maybe not the prettiest solution but I up and running and survived an initial upgrade.
Hope this helps someone else
I got this working. I re-installed to 2 x drives connected to the onboard controller and re-did the procedure to create the bootable USB drive. I then detached the drives and the system had an entry for proxmox in the boot menu this time. I reattached the original drives and had to do a zpool import rpool. Just to be sure I went over the usb creation again just to be sure its UUID got added to /etc/kernel/proxmox-boot-uuids
Maybe not the prettiest solution but I up and running and survived an initial upgrade.
Hope this helps someone else