I found that the file system was the problem for me. Luckily it was a brand new usb hard drive so wasn't an issue to format it.
I first formatted the hard drive to ext4
sudo mkfs.ext4 /dev/<usb disk> -L <diskname>
Then I mounted the usb hard drive
sudo mount /dev/<usb disk> /mnt/<my mount folder>
Once mounted I changed the group ...