I - think to - have successfully installed Qubes OS 4.0.3 on an external USB disk.
I used a 16GB USB drive to write the ISO to, booted from it and then installed it to a different USB drive of 64GB.
I got some installation success message, but the system did not reboot, it was hanging with a black screen and some “The system is now rebooting” or so message.
The newly installed system in fact was not able to actually boot on starting from scratch.
Now the system boots - but not completely: I can’t get an X system up. I see lots of boot messages and then at some point there is a “The SESSION_MANAGER variable is undefined” error and then a series of other error messages (can’t copy paste) indicating that it just can’t start the X Window system. After several retries it then stops trying and just hangs.
Can’t enter some other tty with Ctrl-Alt-Fx for example.
I run Qubes on a USB3 stick and it works pretty well for me, but only when plugged into certain devices. With others I get errors similar to yours, or indeed other errors. What laptop or PC are you plugging the USB stick into?
I’d have thought a Dell XPS would be OK.I’m guessing you’ve fiddled around with the BIOS setings, UEFI etc, see if that makes any difference? Other possibility might be a dodgy USB stick. QubesOS is fussy