Question: how to copy files from linux to usb external drive?
Answer: List the Mount device: lsblk.
Create a mount point : This needs to be mounted into the filesystem somewhere. ... 
Mount! sudo mount /dev/sdb1 /media/usb.
Copy rsync -av /home/android/Testproject/ /media/usb/
5.Un-Mount. When you're done, just fire off: sudo umount /media/usb.