Windows 10 comes with a feature called Fast Startup built into it, and this feature is enabled by default on all Windows 10 computers out of the box. Fast Startup does exactly what its name suggests – it cuts down the amount of time it takes for a Windows 10 computer to boot up after it has been shut down completely by at least half, making starting up after a shutdown even faster for a Windows 10 computer than waking up after going into Hibernate mode.
Fast Startup manages this immense feat by saving an image of the active Windows kernel and all loaded drivers to the hiberfile (the hiberfil.sys file located in the root folder of the partition of your Hard Disk that houses your installation of Windows and the very same file used by Hibernate to save the active session) before the computer shuts down. The next time the computer boots up, Fast Startup simply loads the contents of the hiberfile back into the computer’s RAM, resulting in an extremely quick boot.
Because the hiberfile is necessary for Fast Startup to be able to work its magic, Windows 10 computers that have Fast Startup enabled don’t unmount their Hard Disks’ primary partitions when they shutdown. This can prove to be quite a problem for users who dual-boot Windows 10 with another Operating System as the partition that Windows 10 doesn’t unmount on account of Fast Startup when the computer shuts down can’t be accessed in the other OS because it is still mounted in Windows.
The only way to solve this issue is to sacrifice Fast Startup and disable it, thus resulting in all Hard Disk partitions being unmounted when your computer shuts down. To disable Fast Startup, all you need to do is open the Control Panel, switch to Icons View, click on Power Options, click on Choose what the power buttons do in the left pane, click on the Change settings that are currently unavailable link (and provide confirmation of the action of your password if prompted to do so by UAC), uncheck the Turn on Fast Startup option under Shutdown settings to disable Fast Startup and then click on Save changes.
However, unfortunately, some Windows 10 users experience an uncanny problem where their computers do not unmount the partitions of their Hard Disks that Windows is installed on when they are shut down even with Fast Startup disabled. While this issue is a rare occurrence, it is an extremely significant problem. Thankfully, this problem can be fixed by simply disabling the Hibernate option, resulting in the deletion of the hiberfile. While this solution does require you to sacrifice Hibernate, the deletion of the hiberfile will not only free up a significant amount of Hard Disk space but, with no hiberfile for the feature to use, will also disable Fast Startup for good so that Windows 10 successfully unmounts all partitions every time it shuts down. In order to disable Hibernate and delete the hiberfile, you need to:
Right-click on the Start Menu button to open the WinX Menu.
Click on Command Prompt (Admin) to launch an elevated Command Prompt.
Type the following into the elevated Command Prompt and then press Enter:
powercfg -h off
Once the command-line described above has been executed successfully, Windows 10 will shut down completely every single time you shut it down, making sure that all Hard Disk partitions –including the OS partition – are unmounted every time.