Code:
modprobe usb-storage     ## This is only needed once after each reboot                                          
mkdir -p /tmp/usb        ## The -p tells mkdir not to complain if /tmp/usb already exists                                         
mount /dev/sda1 /tmp/usb                                        
cp -v /etc/rootcert/*.der /tmp/usb/   ## The -v means "verbose", showing exactly what gets copied                     
umount /tmp/usb                                                 
sync