Why is section C no longer compressed, although it seems to be where?

Question

Windows

Note of acquaintances. Hard disk 1 TB. There was only 1 section C (I don’t count any small service ones). It only takes 40 GB. I decided to create partition D for them. Through Disk Manager, I compress partition C - it shows that it can only shrink by half. Well, I think the files are scattered, then we'll fix it. I compress, then I make partition D. I go to defragment C - there is only 6%, in 5 passes the defragmentation goes to 0%. I'm going to squeeze again - he doesn't want a damn thing. Only 10% of the space is occupied in the partition, defragmentation is 0%, and it does not want to shrink. Why is this? I'm interested in the answer to the "why" question. There is no need to compress further yet (until section D is filled to capacity). I just want to understand how it all works.

Answer 1

The size of the volume, through the standard "compress volume" snap-in, will reduce only to the location of non-movable files. Since the MFT table is located somewhere in the middle of the partition, it will not be possible to reduce the system disk by more than half using the standard equipment. Use third-party software by booting an external device. And don't forget the backup.

Answer 2

make a backup copy of the data to another media and after booting Acronis/Paragon/etc software from the LiveCD, make partition changes

Answer 3

compressing the system partition is a perversion, to access the file system you need to extract them and then compress them again, this affects the launch of applications and constant access to the screw