All the Earnhardt's in NC and SC come from 3 brothers who moved to NC in the 1700's. So all of them are related somehow. They had such big families they grew out very quickly.
The various spellings of Earnhardt that are related is Earnhart, Earnheart, Earnheardt, Aaronhardt all coming from the original German spelling of Ehrenhardt which in German is pronounced as "aaronhardt". I am not sure if Ernhardt would be a related spelling, it is possible since the "n" is in it. I do know that Earhart and Ehrhart are not related spellings. There are many German names similar to Ehrenhardt which are not related.