The disk contains an unclean file system

Mounting an external disk you would also use with Windows 8, 10 or newer you might get an error that looks somewhat like this:

Ubuntu error
The NTFS partition is in an unsafe state
Error mounting /dev/sdXY
Exited with non-zero exit status 14
The disk contains an unclean file system
Metadata kept in Windows cache, refused to mount

The reason for this might be the Windows hibernation feature. When you shutdown Windows the services are not stopped but instead are hibernated and pick up were they left the on the next boot. Only when you choose 'restart' everything is really stopped.

You could disable the hibernation feature in Windows:

1. Control Panel
2. Hardware and Sound
3. Power Options
4. System Setting
5. Uncheck "Turn on fast startup"

You could clean-up the NTFS partition from within Ubuntu:

sudo apt-get install ntfs-3g
sudo ntfsfix /dev/sdXY