Just recreated
@presta's experiment using a close to 3 Gbyte file to three USB storage devices:
Sandisk USB stick - not their 'extreme' range, but I thought it was USB 3. Disappointing speed.
Inconsistent speed too, although a strangely regular pattern. It also failed at 99%. Several times (apparently the device "does not exist").
USB connected HDD. Faster than the sandisk stick, and much more consistent.
And with a newer USB stick; fastest of all, but why the hell did it slow down between 55% and 65% ? Maybe there was a power-saving setting; I should have wiggled a mouse.
After careful consideration of the entire thread, and with my cumulative knowledge and experience of using computers for over 40 years, I conclude that USB sticks are
weird.