I have recently installed ubuntu on a 1TB external hard drive, without any special partitioning, I used the express partitioning setting in the installation. Everything worked just fine as I was able to boot from the external hard drive and use ubuntu, and when I unplugged the hard drive from the computer it couldn't boot of course.
The problem is, when I plug the hard drive to another computer, and I have tried 3 different computers, they won't boot from it even when I change the boot order correctly in the BIOS, or I get a "no operating system" error.
I would really appreciate any help. Thank you !