I have an external hdd with multiple operating systems on it (Ubuntu, Centos, etc). On newer computers, the drive is recognized as a USB HDD and I can boot it directly just fine.
However on an older computer, the BIOS allows me to boot from a USB key, but not a USB HDD. Go figure. The BIOS cannot be updated. Therefore I want to use the USB key like an intermediate BIOS, to redirect the boot to the USB HDD.
The bootloader on the USB key is UNetBootin but I cannot find the showcustom commands to boot to the external HDD. I don't need to install anything, I just want to find a way to boot to the Ubuntu partition (or any partition) on the external HDD.
Alternatively since the USB key boots into its own Ubuntu just fine, and sees all the partitions on the external HDD, can I somehow point to a partition and reload it?
I don't have grub