BajanK The part about sending you back to the Main Menu is very odd, unless of course you're exiting to the Main Menu rather than closing the game outright. I'm not sure exactly what's going on here.
To cover all the bases, please remove the Mods symlink and play without mods or custom content, at least for the moment. Play with your computer offline, and let me know which problems are still present.
Please also list the folder the game is currently reading, and the location where it's installed, as listed in the EA App, under Sims 4 > Manage > View properties.
As for the activity on C, it's a mechanical drive, so it's a lot slower, relatively speaking, than the capabilities of your other hardware. But this should mostly only be a problem during loading, not necessarily in the run of play. Please let me know whether it's still at or close to 100% activity once you've loaded into a lot and are just letting your sims go about their business.