you might want to consider to uninstall and re-install Cubase, pointing the installer to a secondary HD for the Content (like your D: drive). The Cubase application itself is not really big and should be installed on the local drive, as Martin correctly wrote, installation of the program on D: is not supported.
Although upgrading to a bigger SSD might be worth and a good choice for the future, I find my whole audio installation using less than 40GB on my C: SSD - I have Cubase, stand-alone programs, including (algo) synths, their presets and a few apps unrelated to audio on the C: drive. Large libraries (Steinberg, Toontrack, NI, IRs and samples) are installed on on my D: drive.
You just have to pay attention to this: when installing Cubase, you can click on every entry on the left (Padshop, Groove Agent, HALion Sonic) and change the installation path. Check that each every entry is set to the same path (e.g. D:\Steinberg Content) and double-check that every entry is set to the same path after making the assignments.