My hard drive failed, so I decided to install Ubuntu on external HDD for now.
I used the Etcher app on a Mac laptop to copy the Ubuntu 18.04 iso to a USB flash drive. This boots up fine.
Then I booted up my desktop with this Ubuntu USB flash drive, and USB external HDD drive also plugged in. I installed Ubuntu on the USB external drive, no problem.
But BIOS will not boot from the USB external HDD. I'm on my desktop, which has run Windows and Linux before (but not tested from external HDD before). I used a Mac laptop to create the Ubuntu flash drive. I tried marking the first partition as bootable, but it didn't work. I tried setting up my own partitions with the first partition as bootable.
I'm not sure how to make my BIOS recognize the external HDD as bootable.
M4A89TD PRO USB3.
EC BIOS Version: MBECB-0012
Western Digital 3TB external USB.
USB flash drive boots up Ubuntu 18.04 desktop version.