NotKnowingAnyThing, You are correct, the H710 does not support non-RAID configuration (JBOD); that is why the 2TB storage was not visible to Windows. Theoretically, the system does not support 2 internal controllers but they are known to work fine. Based on the system Internal RAID PCIe Slot priority 6,4,5,3,1; you will have to install the H310 controller on slot (bus) 4. Before any connections, check in OMSA to make sure  that the 2 backplane channels can separately be connected to 2TB and 3TB storage drives. Let me know if you have any questions.
I could please do with a quick bit of advise.  I have a T420 with 8 bays loaded with 4 x 3TB hdd in a raid 5\ HW driven via PERC H310 (Not much data on here) and 4 x 2TB HDD filled with data.  The 2TB drives until now have been using windows storage pools via windows essentials, there is no VM on this server.  There is no back up of the data on any external storage. I know before you say anything more. :emotion-1: When I've change the PERC for H310 to H710 the 3TB raid 5 shows but I can not see the 2TB storage pool in the PERC bios or in the dell lifecycle and windows is fallijg to boot. I can only guess that the H710 dose not support JBOD there for windows can no see it ever stopping it for booting. So to solved this I was thinking that I could maybe install the H710 on bus 6 of the motherboard (where its meant to be) and run half of the black plate with the 3TB drives in and then plug the H310 into a second PCI-e and run the other half on the black plate just so I can transfer the data across while in windows. Do you think this would work just for a data tranfer? also would you configure you storage diffidently? I an still working on a budget to get more 3TB.   I could set up a second storage of some sorts if i really need to but this would take a very long time to transfer the data across twice so would like to add void this if possible.  I'm only a home user with very little experience of dell servers, big learning curve for me. Thanks for your time on this in advanced.  
T420 h310/H710