Hi Rick,  The onboard ports shouldn’t have a limit, but we haven’t tested newer drives over 4TB. 
I picked up a fully functional R310 from a university salvage store for $60 and made a home file server. It came with two 500 GB SATA drives cabled to the SATA ports on the motherboard, and no RAID card. I fully updated the firmware and loaded Windows 10. It's running stable and everything seems to be fine. Now it's time to replace the 500 GB drives with new WD Red+ drives. Do I have a size limit on drives directly cabled to the motherboard SATA ports, like the 2 TB limit when going through one of the stock RAID controller cards? I'd just try a 4GB or larger drive but don't have any that aren't holding data on another system.  
R310 Motherboard SATA Drive Size Limit?