Comment 3 for bug 1557894

I have discovered the "strange" device format of the image is a hybrid iso, and gparted simply does not recognise this as meaningful. I was able to mount the iso as follows:

  sudo mount -o loop ./image.iso iso-contents/

... and then the mounted directory allows me read-only access to the file system within the image.

As I said above, the Asus EFI CSM BIOS detects and executes the hybridiso's bootloader. However, something goes wrong further into the process. Perhaps this is because the hybrid iso's file system also contains efi files and their presence confuses the BIOS. I will continue to investigate.