Corrupted CC

bettybowditch​  It sounds like everything is in fact corrupt.  I'm not sure why that happened, except perhaps the transfer to the external didn't go smoothly (or took more time than you thought it would) due to the sheer volume of data.

Do you have copies of anything left on your internal drive?  If so, you could transfer the Mods folder again, this time zipping it, or better zipping one subfolder at a time, before moving it.

If that's not possible, one thing that might help is running some of the cc through Sims4Studio batch fixes.  These are not meant to fix corrupt cc, rather cc that gets broken with a specific patch that updates how the underlying item works.  But it's possible that a batch fix would repair your issue too.

Other than that, there's nothing you can do except start over, unfortunately.  It might be easier to only download what you're actively using, for now, and go on a cc shopping spree or two down the road.