I have made much progress with my Rpi 2+ using the
new OS stretch. I used a usb disk to bring some
files to the Pi. I did something I don't usually do
and just pulled the disk out. There was a was a warning
to use the file manager, I think, to umount the disk. I
can't find such a tool on file manager. I know I can
use the umount command to do it but on Xubuntu 16.04 you can
indeed umount the disk in the file manager.