In this case, the immovable file is actually the MFT, or Master File Table for the volume.
At this point you should be able to shrink the partition, at least a decent amount.
Using Linux to Resize
you’ve got the option to also use thegparted live cdto resize your partitions.
We prefer free software here whenever possible.
In summary, If you are lucky enough for Shrink Volume to work for you, great.
If not, I’d recommend backing up, wiping, and installing fresh.
