Hi placrosse
That sounds like the USB stick was not found. It should have said something like /mnt/sda1
Enter fdisk -l and see if the USB stick shows up in the list, you should be able to spot it based on its size.
If its not there, try unplugging and replugging the device, wait ten seconds, and run fdisk -l again.
Note which sd value is listed where it says /dev/
Enter mount /mnt/sdXX where sdXX is the value you observed in the previous step.
Then cd /mnt/sdXX , you are looking for a directory called syslinux, it might be under boot/. In that directory there
should be a .cnf or .conf file. Open the file, find where it says waitusb= and change the number after the = sign
to a larger value, that's the number of seconds it waits for the device to show up. Save the file and see if it boots now.