@mountainlover The HDD should mount on its own... It'll attach to /media I believe...
If you hard manually mounting it - via the mount command, can you unmount via umount?
Also, you may be getting the cannot eject in use message if there is something open to it, file manager or another application: Backup, Viewer, Reader, etc etc...
If you cant eject, for what ever - you can shutdown the pc, that should close out any open/held connections.
While its plugged in and working...
Perhaps the output of... would be a start...
df -h
lsblk