A more complex but more commonly used program is rsync

rsync -rav /home/user/Documents /mnt/usbdrive is treated differently than rsync -rav /home/user/Documents/ /mnt/usbdrive which is different than rsync -rav /home/user/Documents /mnt/usbdrive/ which is different than rsync -rav /home/user/Documents/ /mnt/usbdrive/

It's a great tool for making copies onto drives, even servers. But man you have to double check how each folder path is laid out, otherwise it'll write the files of one folder to the main drive, unorganized.