Lost 516GB of free space after compression (rather than regained it)

[realNZZN]

edit: This issue was first documented over a year ago #325

I ran CompactGUI (LZX mode) on a backup drive to compress a folder of fairly large (but mostly empty) disk images.
While the files were being 'compressed', I noticed that the free space on the disk was slightly decreasing, but assumed it was normal and the that disk space was being used as a working space for the compression. After each file finished compressing, the newly occupied disk space was not cleared, and when I got down to just 10gb free (I started at 120) I clicked the cancel button, which now showed "Cancelling..." but nothing happened and I kept losing disk space, so at 5GB I shut down windows, which got stuck on the shutting down screen so I manually powered off the PC.
After booting windows again the disk still showed 5gb free, so I deleted some files to be on the safe side. While windows reports 907GB of the disk as occupied, it also claims the total size of the files (including hidden files) on the disk amounts to 587GB, or 391GB "compressed"

This is a pretty awkward question to explain, so I hope the image shows the issue I'm having

WinDirStat, System Ninja, and Disk Clean-Up are unable to find where this 516GB has gone so I'm not sure what I should do at this point since this disk was being used as an active backup drive for multiple systems.
The Uncompress option in CompactGUI gets stuck as it did earlier and I have to power off the computer, and while File Properties shows every file on the disk as uncompressed (The compression checkbox is disabled), trying to compress them causes Explorer to hang waiting for an invisible error window.

[Cregrant]

My suggestion is to scan the disk through defrag software to find where the free space went (Auslogics Disk Defrag shows even MFT files but can't defrag compressed ones), delete those files, and run a disk error check in `Disk Properties -> Tools'. Maybe run error check first if this is a MFT issue.

[datkush420]

I'm having this same problem now, did you ever figure it out? I canceled during re-compression because it seemed stuck. TreeSize reports 198GB/238GB used, but Windows tells me that I only have 4GB free. I should have 40GB free. Where did it go? Disk Cleanup and Defrag did not change anything.

Edit > Disk Properties - Tools: No drive errors and no change of free space. Auslogics Disk Defrag (free version) no bueno either.
Edit2 > Reinstalled the affected game & launched it so it could do shaders and whatnot.. ended up with 82GB free. Windows is weird. All is well it seems.

[Iridium-IO]

I spent a few days looking around but never did find a clear explanation or fix sorry. It's always hardest when they're issues I can't replicate on my end