My boot drive is a 120 GB SSD (actually 111 GB). Windows reports that I have 15.5 GB used, meaning I've used up 96.1 GB. I know that's wrong, so I ran windirstat and it reports that I've only used 68.5 GB (despite agreeing with windows that I've only got ~15 GB free), which I know is right. Now when I originally had a problem like this when I first built my computer, it was my pagefile and hiberfil. I've sense deleted those. My recycle bin is also empty. So I'm struggling to find out what is causing this discrepancy. I'm fairly certain it's not my system backup since i've got that saving to an external. I'll still pursue that for the time being, but I'm curious if someone can shed some light on my predicament here. Thank you!
The other place where disk usage can "hide" is system restore.
Also try rebooting. I've noticed on my laptop with SSD, deleting files does not always free up the space. I presume there's some sort of delayed TRIM function going on before the deleted space is marked as free. Rebooting seems to force that to happen.