Once upon a time I was installing Linux on a tiny little laptop, whose brand name I've forgotten. It was probably a Lenovo. Anyway, it was extremely difficult to install anything on it, and they went to great lengths to make sure no one would be able to install Linux on it. I spent an entire day messing around with the grub terminal, and began to suspect that it had a built-in cut off for the USB port during boot. I think I saw some log output to that effect, but I couldn't find any way to disable it. After some thought, I got back in grub, unplugged the USB stick that I was installing Linux from, and plugged it back in. The laptop detected and mounted the external drive and I tried to install again.

Worked perfectly.