USB Hard Drive Size Limit

Is there a limit on how big the external HD can be to use with the two USB ports? Is 4TB the limit, or could I connect a external USB 3.0 HD that is 6TB and use it without any issues. 

I tried to connect a 3 TB WD Mybook and it didn’t recognize the drive.

Interesting! I have had no issues with my 3TB drive I got from WD. I was just looking at ideas of what to do once I fill the 16TB up(which is happening sooner that I expected). Utilize the 2 USB ports to add an additional 8-10 TB or invest HEAVILY into synology(which I would love to do since I hate the MYCloud line and its lack of abilities.

Though to be fair was surpirsed to see 2 software updates in less than a month; even if the second was a minor fix!