windows - Why is my NTFS parition full when it's not?

My D: drive of 137GB (NTFS) would appear to be full, but I personally disagree. This are the reasons I disagree with reality:

1. Selecting all files and viewing their properties, it says 43.4GB size and 43.8GB size on disk. Because the size on disk is nearly identical, I don't think I have a million tiny files filling up sectors.


2. In Explorer I show hidden files and also protected system files, so explorer includes as much data as possible when counting the above size and size on disk.


3. Viewing the partitions' contents using 'SYSTEM' permissions (which are above 'Administrator'), there are no huge files in System Volume Information and no other invisible directories.


4. SMART has always been enabled and is currently not showing anything weird.


5. The partition/disk does not have anything in the read/write cache. I've used sync and, after that didn't work, also tried rebooting. I didn't do a cold boot (that sometimes tends to help even if rebooting doesn't fix something. Should I try that?).

The reasons that make me think the disk is full at all is that Windows cannot create new files (from explorer nor any scripting/programming language), and because when viewing the partition's properties it reports to have used all 137GB.

What might I have missed? Where did all my free space go? How can I view what's taking up all the storage space and get rid of it?

When cleaning stuff up, space is indeed freed again and I can reoccupy it with other data. I've also recently cleaned up a directory with ~160 000 small (80 byte) files, which did clear up a couple of megabytes (though I'd expect it to clear up 160000*4KB (sector size) instead of the 160000*80B which it did), but I still can't find anything that might occupy two thirds of the entire partition.

Unfortunately I'm talking about a server and I don't have the luxury of owning redundant hardware, so I'd prefer not to have to boot from a live stick if in any way possible. I guess I could do some scheduled nightly maintenance if there is really no other way to inspect or repair things properly, but I'd like to keep that to a minimum. I am somewhat experienced with GNU/Linux so I'd know my way around on a live boot command line if you tell me which commands/tools I should check out.

Edit: By the way, NTFS compression is not enabled.