I have been looking at other threads. Some have said the problem is a permission issue, and some have said it hasn't been solved. I am using rsync to backup to a USB drive. When I insert the usb drive, I can find it in /media/pi/USBNAME. Here is the command I'm using.
sudo rsync -avH --delete-during --delete-excluded --exclude-from=/home/pi/Documents/backup/rsync-exclude.txt / /media/pi/*/
It works fine for a while, but then it will randomly stop copying the files to the USB drive. If I re-run the command, it will stop at a different file.