Thanks for all good advice here. You have really looked into this matter. The only thing I have noticed was a big difference between USB-flash and USB-HDD for copying. What I counted in hours for the flash I counted in minutes for the HDD.
(When using a USB-installation I guess the amount of RAM is significant? The more you can cache the quicker you can work.)
EDIT: I had better add it was mostly for copy/WRITE of large amounts of data.