Can I safely resize Windows 10 system partition from Ubuntu?

Solution 1:

The safest way is to use Windows 10 to shrink it's own partition.

Windows 10 knows more about the last used area of the disk partition. Ubuntu / gparted cannot shrink the partition reliably as Windows 10 can.

For example last weekend I wanted to clone 16.04 and do a test run upgrade to 18.04. My Windows partition was 411 GB with only 100 GB used. Windows 10 would only shrink by a maximum of 25 GB because it saw used files near the end of the partition. If any more was needed Windows 10 would have to defragment the disk.

Solution 2:

Just in case anyone finds this in 2020 like I did, this is my experience.

I had Windows 10 installed on the left half of a new 1TB SSD and just a plain old ext4 filesystem (no OS) on the right half. Very new Windows 10 installation and very ample room.

I tried using the Windows Disk Management to shrink my partition, but as many other folks will agree, it said it would only shrink it by up to 2GB or so. I ran the graphical and the command-line defrag, but that number didn't change.

Then I booted from a Ubuntu 20.04 live USB and used GParted to delete my ext4 partition, shrink the C: partition, then move the Windows Recovery partition to the left to fill in the space. The last step gives a scary warning, which you can just click through. I ended up booting Windows in between each step just to be safe, and there was not a single hiccup the whole way through.

I was very pleased to see that Windows 10 seems to have matured and doesn't throw a fit when things change.
Bottom line: GParted works perfectly with Windows 10.