Linux mint / windows 10 dual boot - unable to access windows files (SOLVED)

The solution I found that worked was a program called ntfsfix.

Install ntfs-3g with:

sudo apt-get install ntfs-3g

Then run the ntfsfix command on your NTFS partition.

For example:

sudo ntfsfix /dev/sda4

Source: https://askubuntu.com/questions/806570/cannot-access-windows-partition-from-ubuntu