Which key has to be pressed to enter a boot device selection? If there is none which boot order has to be used in the BIOS in order to boot from an USB stick (for backup purposes for instance) when it is attached to the notebook?
It’s the delete key (to enter BIOS). I don’t think there is a separate boot device selection screen like in some other BIOS’. In the BIOS you can go to the rightmost tab (the one where you can save & exit) and at the bottom you will see the list of available boot options. Just select one of those and press enter to boot from there.