Comment 5 for bug 1970995

I just learned this is an old bug,

https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1396379

with a suggested workaround here (basically, temporarily disable the boot, efi, hidden flags of the internal drive):

https://askubuntu.com/questions/16988/how-do-i-install-ubuntu-to-a-usb-key-without-using-startup-disk-creator/1056079#1056079