In Windows 10 Preview, every time when upgrading to a new build release, Windows will automatically keeps a backup of its existing OS prior upgrade so when things go wrong with the installation you can always have the option to rollback to the previous release. The backup is stored under Windows.old folder of your root Windows OS partition. While this is a welcome feature by Microsoft, you are also losing disk space ranging from a few GB to 25GB, which could be very precious on a Tablet like Surface or a SSD drive.
To remove the Windows.old directory you need to go to Disk Cleanup and remove it from there by selecting “Previous Windows Installations”. But for some reasons in Windows 10 (technical preview), the first time when you go to Disk Cleanup this option is nowhere to be found.
Guide To Remove Windows.old on Windows 10
As you can see my Windows.old directory uses 15.2GB of actual space on my hard drive. It’s not safe to simply delete the folder from there, and most likely you will not be allowed to complete this deletion. The proper way to remove this directory is through “Disk Cleanup”, a built-in utility from Windows that manages any system file deletion.
Right-click your C: drive, go to Properties > Disk Cleanup
As you can see from the screenshot below, I was puzzled that why there are so few options available in the Disk Cleanup utility. Clearly the “Files to delete” section doesn’t contain the option for previous Windows installations.
But that’s OK. Just click “Clean up system files” anyway, which will run through the disk cleanup scan process again. Once that’s finished, you will see more options available to choose, one of which is the ability to remove “Previous Windows installations”.
Select the Files to delete
A warning will prompt when removing previous Windows installations, you are going to be informed that
If you clean up the previous Windows installations or temporary installation files, you will no longer be able to restore the machine back to the previous version of Windows. Are you sure that you want to do this?
Click Yes to acknowledge this warning and proceed.
Once the process is finished, you will gain all the valuable space back! Hope this helps you when navigating around the new Windows 10.
/Update on Mar. 10, 2020/
You can clean up previous Windows installations from the Settings app as well now.
Go to Settings > System > Storage and click Temporary Files.
If there is a Windows.old folder you will see an option called Previous Installed Windows that you can check to clean it up.