Re: playing the sims 3 on macbook pro with external hard drive

More than likely it's the disc and not the drive.  This is a common error with the game.  I have a Mac too so yeah I'm familiar with it.

There's this thread on the topic: http://forum.thesims3.com/jforum/posts/list/30/482235.page

This help article from EA, but you should be careful with some of these "fixes" in the Mac section: https://help.ea.com/article/what-steps-can-i-take-to-attempt-to-correct-a-disc-authentication-error

If I were you, I would try repairing permissions first, if you haven't done that.  The usually helps for me.  The steps for that are in the second link. 

edit: and by disc, I mean the game itself...not that you need another disc.  Some people have tried exchanging for a new copy only to have the same error.