Of course, no worries
Ok your C:\ has all your operating system, programs and possibly Cubase library content on it at the moment.
Currently whenever cubase wants to access a file, whether it’s a plugin or audio file, it has to load it all from the same drive(don’t forget also your operating system and any other apps you have open will also be accessing it at the same time). If you are working on projects that have a large number of audio files and possibly even sample library (don’t forget that many synths/drum machines etc are sample based) then cubase will be having to access that drive constantly, on a hard drive this can cause it to run out of bandwidth and you can get audio dropouts or it can even crash that hard drive… i’m not sure what the current case is with SSDs though.
So spreading the load over two or more drives gives your system much more headroom where this is concerned and less chance of a drive failure, although this is much less common than in the past.
If you buy an external drive i’d check with some of the guys on here as to which are good for audio use, USB3 or Thunderbolt should be the quickest.
MANY of us on here use multiple drives in our systems… i have C:\ for OS and programs, D:\ for audio, E:\ for sample library for things like BFD 2/3, Halion, UVI etc. I also have another couple of drives, one for system backup and one for my HUGE porn library (joke)… i mean music collection and various other bits and pieces.
I also keep a system image on an external drive just in case.