[ubuntu] Booting from external hard drive

I installed ubuntu 16.04 on an exteal hard drive. There is nothing else on the drive. I wiped it with the install.
I have 4 old computers ruing various versions of linux.
On each computer I plugged in the hard drive, adjusted the bios so the 1st thing it would try to boot from was a usb device, and booted up.
On one computer it worked fine. This was not the computer I used to install ubuntu to the exteal hard drive.
On each of the other 3, there were 3 different error messages, but none of them would boot.
On each of the other 3, if I booted from linux on the inteal drive, I could see the exteal hard drive and read and write to it.

Is this common? Are there often problems with booting from exteal hard drives?

The exteal drive is a
WD My Passport Essential 500 GB USB 3.0/2.0 Portable Exteal Hard Drive
https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

I could go into more detail about the error messages on the 3 computers where it would not boot, if that would be useful.