steamapps prefers to be on C:\ drive so its always muddy water trying to split steams load onto another hard drive. i would say porch other games non steam related onto your hard drive to free room for steam instead. if you must split the load, i think you can just drag and drop your games files into the steamapps when you want to play them from the external but youd wanna not be running steam when you do it start steam after you do it and then you may still need to reverify your files after doing that through steam if you have glitches or issues. and changes to the folder system of steam in C:\ that takes place steam must be restarted unless its steam working to edit files within itself itself. or maybe even creating its own new files.

in copying and pasting you would want any full folder system within common/ folder or sourcemods/ i dont think youd want to transfer or tamper anything else from steam folders. mods use the source sdks so if you remove source engine stuff itll remove all the source games so as far as source goes you would want to just only move those common games folders or sourcemods games folders. u wouldnt wanna move core steam apps or tool steam would look for those by default where there suppose to be but you could also check settings for the games there may be a way to reconnect or repoint the game to a different folder but thats where things would get messy for steam probably. alot of the folders within are shared amongst ALOT of games.