I was worried today that the SSD I installed Windows on was too small. I had hardly installed anything but the OS on it but all but a couple of gigabytes of the 64GB drive were occupied. Upon further investigation, I found that Windows itself only takes up about 15GB and nothing else on the drive even exceeded a couple of megabytes, meaning one of two things: the drive was defective, or there were big, invisible files taking up a lot of space.
Fortunately, it was the latter case - a couple of well-hidden files called "hiberfil.sys" and "pagefile.sys" were the culprits. They're both files that vary based on the amount of memory the computer has, so with this machine's 24GB they occupied a good forty or so. pagefile.sys had to be moved to the 7.5TB drive where there was plenty of space for it, and disabling hibernation (something I don't think is often used on desktops anyway) got rid of the other one.
So now the boot drive has plenty of free space.