What Gane O'dwyer stated is correct, unless you get issues with a slow-indexing problem.
I do have a 8TB harddrive with 3.5TB files on it. The problem is, that while syncing, indexing also starts.
If you have the files already on your harddrive like I do, then it should only index if the file hashes are the same.
But, indexing 3.5 TB is already taking 2.5 days, and seems to be getting slower every hour.
So, it should be quicker, but that is not always the case depending on your situation.