The one and only purpose for the Windows.old folder is to allow users to revert to the previous Windows version. This folder will be automatically deleted if the user does not revert within 10-day period following either a public release version upgrade, or an Insider build upgrade. Personally, I have never understood its raison d’etre, and always remove it immediately after an upgrade. Let me explain why.
What is Windows.old?
The Windows.old folder is created when a feature upgrade starts its offline phase, after the user clicks Restart now. The Windows setup process copies Windows system files, program files and program data (“all users appdata”), and user data including app data folders to the Windows.old folder. The folder structure in Windows.old is exactly the same as in a default Windows 10 installation.
When a user chooses to revert to the previous Windows version, the contents of the Windows.old folder are copied back to the root of system drive C:, overwriting updated / upgraded files and folders. Windows.old\Program Files is copied to C:\Program Files, Windows.old\Users\Kari\AppData\Roaming to C:\Users\Kari\AppData\Roaming, and so forth and so on.
Here’s a simple analogy: you could think of Windows.old as a shadow copy or image backup, its only purpose being to restore Windows to its previous state.
Why I think it’s totally unnecessary?
As you know, Microsoft has deprecated the Windows built-in system backup and imaging, and itself recommends using third party imaging tools. Quoting from the Microsoft Services Agreement, I find this text:
6. Service Availability.
a. The Services, Third-Party Apps and Services, or material or products offered through the Services may be unavailable from time to time, may be offered for a limited time, or may vary depending on your region or device. If you change the location associated with your Microsoft account, you may need to re-acquire the material or applications that were available to you and paid for in your previous region.
b. We strive to keep the Services up and running; however, all online services suffer occasional disruptions and outages, and Microsoft is not liable for any disruption or loss you may suffer as a result. In the event of an outage, you may not be able to retrieve Your Content or Data that you’ve stored. We recommend that you regularly backup Your Content and Data that you store on the Services or store using Third-Party Apps and Services.
In my opinion, that recommendation should also be applied to Windows feature upgrades. Any user who does not make regular backups is an idiot (I am not even the slightest sorry to use such a strong word). I really, deeply and profoundly think so. As I see it, the only method to revert to the previous Windows version that makes sense is to create an image backup using any third party imaging tool before the upgrade. Then, if reverting proves necessary, restore that backup.
I have two arguments to support my point of view. For the first, using my HP ProBook 470 G5 laptop (tech specs) as an example, reverting using Windows Settings > Update & Security > Recovery > Go back to previous version of Windows 10 takes over 50 minutes, sometimes even over an hour. OK, I admit, I have a lot of stuff in my Windows installation, and the Windows.old folder can be up to 35+ GB. The smaller the Windows.old folder, the faster it will be to revert.
But, creating an incremental Macrium Reflect backup before starting the upgrade process, and restoring it if needed, the total time required is in my case less than 20 minutes.
Second, Windows.old takes a lot of storage space. As I just reported, my Windows.old folder can be 35+ GB, and that’s on a 128 GB SSD. This is space I cannot afford to lose (or waste), even for the 10-day grace period. In addition, think a bit about what the globally known tutorial guru Shawn Brink says on his Ten Forums tutorial:
You will need to have enough free space on the Windows C: drive. Usually the free space needed is at least twice the size of the C:\Windows.old folder.
1.) Create a system image backup before starting the upgrade process.
2.) When upgrade is done, run the following command from an elevated Command Prompt:
cmd.exe /c Cleanmgr /sageset:65535 & Cleanmgr /sagerun:65535
This opens Advanced Disk Clean-up. Select everything, especially Previous Windows installations:
This cleans all disks, internal and external, and removes the Windows.old folder.
3.) If reverting to previous build is necessary, or if Windows upgrade fails, boot from your imaging tool’s boot media and restore the backup.
In my opinion, the whole process of creating the Windows.old folder and using it if the user decides, for whatever reason, to revert to the previous version is completely unnecessary. The same if the upgrade fails. It’s faster, and more practical, to use any third party imaging tool to do the same thing. It would also save a lot of space on the system drive if the folder were not created. Better yet, this would make the upgrade process faster, too. As the Germans say “Raus mit dir!” (literal English meaning “out with you!” best rendered as “Get it outta here!”).
Author: Kari Finn
A former Windows Insider MVP, Kari started in computing in the mid 80’s writing code for VAX / VMS systems. Since then, he’s worked in a variety of IT positions. He specializes in Windows image capture, customization, repair and deployment as well as Hyper-V virtualization. Kari is a proud Team Member at number #1 Windows site TenForums.com.