Can't shrink to available partition volume in Windows [duplicate]

I have 1TB HDD with 800GB free space plus a typical System Reserved partition.

I removed system files before shrinking in Disk Management since suggested shrinking size was no more than 100GB

• pagefile
• hybernate
• system restore
• debug info
• disk clean up

rebooted and performed defrag in c: -> properties -> tools

That gave up to 800GB available shrink size, but still shrinking it in Disk Management to suggested or smaller amount results in

There is not enough space available on the disk(s) to complete this operation

It worked shrinking off small amounts like 100GB

Use GParted. It sounds like you are trying to shrink an actively running operating system. You generally cannot do this unless you have already split this 800GB chunk off into a partition that is separate from the active OS.

Edit for added details:

You will need to use GParted to do the shrinking operation since that partition is the active OS. Also it would be a good idea to leave approximately 40GB or so of buffer space on the Windows partition just in case.