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

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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

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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.

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  • Yes I did as I was confused. I tried with Disk Management and then diskpart. Shrinking to 700,500GB failed but I accidentally typed 200MB in diskpart and it worked with that amount unallocated appeared – Chesnokov Yuriy Jul 11 '14 at 19:32
  • yes I was thinking to finally try from live Ubuntu, hope it would not damage windows partion – Chesnokov Yuriy Jul 11 '14 at 19:35
  • It won't. Worst case you just need to run a chkdsk.exe and then try again. – Enigma Jul 11 '14 at 19:36
  • why it did cut 250MB? I just succeeded with 100GB more – Chesnokov Yuriy Jul 11 '14 at 19:40
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    You might be able to clear more space by using a disk optimizer like auslogics.com/en/software/disk-defrag to push as much data as possible to the front of the disk. Usually the windows defragger does minimal defragging and doesn't optimize fully. The result is that while things aren't fragmented from eachother in terms of usage, you may still have small bits of data scattered about. – Enigma Jul 11 '14 at 19:46

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