Changing drive locations - what needs to stay on C:/

I have a server that is running out of drive space on C:/ and so SEP was installed on the second hard drive.  I know by default that the definitions are saved on the OS drive (in this case C:/) and that I can edit the registry to look for/store them on another drive(in this case D:/).  In looking at the folders on C:/ it appears that more than just the definitions are saved by default to C:/ despite the install directive to install on D:/.  I can copy all these folders over to D:/ and modify the registry to look for/save them there however are there any folders that absolutely need to stay on C:/?