Disclaimer: This is what worked for me, and it’s not guidance from Microsoft. It may not work for you. Since this involves resizing partitions, it could wipe out all of your data. You may want to create a backup first. Proceed at your own risk.
Over the weekend I upgraded my machines at home to Windows 10. I had two desktops and one Surface Pro (the first one) running Windows 8.1. Since I had multiple machines to upgrade, I downloaded the Windows 10 installer to a USB flash drive using the media creation tool mentioned on Download Windows 10. For me, the media creation tool wouldn’t recognize the USB drive, so I chose the ISO and copied the contents to my flash drive. I used the flash drive to upgrade my Surface Pro with no issues.
Then I tried to upgrade my desktops. The first one failed with the message saying, “We couldn’t update the system reserved partition.” That happened after it downloaded updates (that takes a while). I tried my other desktop and got the same message.
I pulled up diskmgmt.msc and saw that my system partition had a size of 100 MB and was essentially completely full – 3% free. Both machines were in that state. So I started searching for a solution. I ran across a couple of places, such as this one on reddit, that had a set of instructions to free up space. It included commands that I didn’t actually know existed (I’ve never needed takeown).
I followed the instructions and had more than 50 MB free on system partition. I ran the Windows 10 upgrade, and this time it got further before failing and failed with a different message (I don’t remember exactly what it said).
At this point I decided to extend the system partition. The thread on reddit mentioned a tool called MiniTool Partition Wizard Free. I did a search and found a review on PCMag. They were complimentary of the pro version, so I decided to give it a try.
The UI makes it really easy to drag the OS partition to resize it a bit smaller and then extend the system reserved partition to make it bigger. I shrunk my OS partition by 200 MB and increased the system reserved partition to 300 MB. After hitting Apply, Windows has to be rebooted for the tool to make the change. I did that and let it do its work, and then all was good. Windows 8.1 booted up just fine with the newly resized partitions.
I ran the Windows 10 upgrade again, and the upgrade proceeded smoothly. Thanks to the folks at MiniTool for a great tool!
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