I always have my Bios set to boot from the HD. Anything else
I touch esc or F10 or F12 depending on the machine. Then
you make a selection which device to boot from, if any.
Anything else, USB or CD, automounts thru PCManfm in
LXDE. etc/fstab could be modified to mount something to
a specific someplace if that is desirable, but not really needed
if it automounts to the same place all the time, always
/media/devicelabel directory. Should be an easy setup
then without error messages.