Do not recall anyone mentioning having problems with large (over 3T) USB hard dives on the My Cloud other than the usual USB not recognized issue that plagued some of us that is not related to the USB hard drive size. The user manuals for the My Cloud (http://www.wdc.com/wdproducts/library/?id=439&type=25) do not appear to list a capacity limit to the USB hard drive size.
Someone asked a similar question over in the EX4 subforum several years ago:
