I've installed Ubuntu 18.04 on a Sandisk Cruzer 8gb flash drive. I used a PNY bootable USB to install from, all running off my MacBook Pro (Early 2015, OSX 10.13.4).
I booted into the PNY, selected the "Install Ubuntu" option in the boot loader, then installed to the Sandisk. I also was prompted to choose a location to install the boot loader, so I installed it on the Sandisk. Everything completed without crashing, and I was prompted to reboot in order to use the installation.
rEFInd doesn't recognize the Sandisk at all, much less as bootable. However, when I go to re-install Ubuntu on the Sandisk, there's already an Ubuntu installation there.
What's going on? Have I missed something or skipped an important step? Any help is appreciated.
/dev/sda, the UEFI bootloader will be put (or merged) into an EFI system partition (ESP) in/dev/sda. You can prevent this, if you can disconnect or unplug the internal drive. Otherwise you can copy the necessary directories and files manually (but it is a little tricky). Maybe the following link can help, Boot Ubuntu from external drive – sudodus May 29 '18 at 05:28