It is counter-productive to keep everything in one Runtime. This will slow down P5 (and almost certainly P6). Make a new Runtime folder on the extra HD and move the sub-directories from the C drive runtime into it. As long as the directory structure of the Runtime is intact, P5 (and P6) can access the content properly. Manage the multi-Runtimes more like you manage normal Windows directories. By all means, break free of the P4 filing habits! Use deeper sub-directories. Access is bogged down by bloated shallow runtime folders. 200 files in ...propsweapons will be much slower access than the same 200 files in ...propsweaponsswords+guns+knives+etc.
This also makes it easier to find the props.