There’s also WinDirStat - pretty much the same as Treesize Free, I just find it faster:
Another thing to check is hibernation and pagefile. Depending on your settings these can take up a significant amount of space…especially on a drive as small as the one you’re using. If your pagefile is 4GB, that’s 5% of your drive gone right there. Hibernation files can be several GBs as well. Tack on Cubase, and Windows, various standard Windows programs, and you’ve probably eaten through 1/2 your drive.
Also important - .exe. and .dll files of various effects / instruments, etc. can add up substantially. My Native Instruments folder (Komplete Ultimate) with only about 1/2 of the content installed, takes up 3GB. That’s .exe files, presets, manuals, and not including .dll files, which take up another 1GB. And that’s not including sample content, which is on another drive. Also watch out for impulse responses from reverbs / cab emulators. Also, sample library content from stuff like Absynth. Most programs can have their data libraries linked to another drive - that includes all of Cubase’s .vstsound sample content. It also includes data files from companies like U-He - these can be put on another drive.
Also watch for programs that install both 32-bit and 64-bit versions. Or VST2, VST3, AU, AAX, etc. versions. There are some companies that install every format automatically. If you are using dozens and dozens of programs, there’s a lot of wasted space.
Also consider installer files extracted to temp, and windows update fragments. If you look at TreeSize or WinDirStat, you’ll notice that MSI / Temp files take up a lot of space (depending on how old your OS isntall is…it will just keep increasing.) Unfortunately, even if most of them aren’t in use, removing these files can potentially cause problems. Especially for uninstalling or updating some programs. For this reason alone it’s usually good to have a larger OS drive with ample free space.
I would also recommend having some kind of scratch disk for downloads. I have a spare 1TB HDD I use for pretty much all downloads - sample library, Chrome browser downloads, etc. All my extracting / installing happens from that drive. I’ve written like 20TB to it over the last year and bit. It gets a lot of use, but it keeps a lot crap off my OS drive (and it means I’m not always extracting huge library files on my sample library SSDs. Everything is set to download to that drive - Chrome, app installers, sample library zips, Native Access, etc.
As a general rule, a drive of your size is going to be a problem no matter what. Most large updates - including windows - are going to require twice the space of whatever is being installed, as it will have to temporarily extract those files before installation. When there’s no space to work with, performance is going to suffer. If it’s an SSD, the nature of SSDs mean that more available space = longer potential lifespan. SSDs really suffer from crammed drives. This is still the case even with the newer NVMe generation.
These days, a 256GB SSD can be had for like $100. I would totally recommend going this route if you have the cash. Will save you soooo many headaches.