I would play it safe and plug it in before you boot. Then open a terminal and enter
it should say something about your USB device and about where it's put it, possibly sda1. make a folder somewhere to mount it. Possibly /home/username/USB ? Then as root issue the command
mount /dev/sda1 /home/username/USB
You might need to change the ownership of the files so you can write to the device.
chown -R username:usergroup /home/username/USB
In the above, change username and usergroup for words that make sense on your system.