Legal Question in Traffic Law in South Carolina

Based on what you described, and assuming there no other important information, then my answer would be yes, it was a valid stop.

A police officer is allowed to stop someone who failed to use their turn signal when required. If you made a complete left hand turn, then you were required to use your turn signal. It may be totally ridiculous from a practical standpoint since the roads were clear of other vehicles. Nevertheless, the law simply says a person is required to engage their turn signal when making a turn. Period. The law does not say the person much use their turn signals only when there are other cars (including pedestrians or other forms of activity) on the nearby roads. So speaking from strictly a legal standpoint, the stop was legal.

You didn't say if you received a ticket or not. I would hope not. It would be an ideal situation for the officer to give a warning. My guess is, and its only a guess, is that you just received a warning. One of the reasons officers are allowed to pull someone over in certain situations and just give them a warning, is when the officer believes the individual truly did not understand the law and therefore didn't violate it intentionally. It wouldn't be a legal defense, but its still an appropriate situation to just advise the driver on the law and then give them a break and not ticket them.

I should probably add that my response is basically professional guess work, since I was not there and didn't see what happened and am not familiar with that intersection.