Install 20 on new Mac usb c fails at Grub step

[kalpana]

I am trying to install Ubuntu 20.04 on a MacBook Pro 2019 external drive connected with USB c.
New Macs like this have known complications for installing Ubuntu.
Made the installation stick with unetbootin. Using refind as boot manager.
External drive is Samsung XS Portable with SSC
Install goes well until installation of Grub. Then just says Fatal Error.

Tried a few times, remaking install stick, etc. Always the same problem.