Well I would make a bootable GParted and that will allow you to Unmount. If you are trying to unmount your drive while your computer is idle it will not let you unmount. But if you make a bootable Gparted it will allow you to have full control over your partitions  the bootable is actually Debian with the Gparted Program along with some other tools.

How to make a bootable link is below from official
Link: https://gparted.org/liveusb.php

1.  In  the  boot process of Deepin , thiere is a option  "Deepin recovery"   , enter this mode, and Gparted can resize your current partitions , then you can create new partition  and install windows. But the installation process might do sth with the bootloader ,so be careful.


2.   Crossover is another method for you to install some of the win programs without win system.