RobC-UK,  I do see that the R220 server does support 6TB drives, such as part number PYM8J, but that is a SAS drive. The largest SATA I see listed is a 4TB, part number GCHH1. That doesn't mean that there aren't any 6TB SATAs  that will work, just that I don't see them listed.   Let me know if this helps.
Hi, I'm looking to get a couple more years out of the R220 II we have.  Hard disk technology has moved on since 2014 and thus I was wondering how big a hard disk I can install into the server.  The documentation says 6Tb but I suspect that was when 3Tb disks were still new.  Ideally I'd like to install a couple of 6Tb 3.5" SATA drives. Thoughts? ThanksRob 
PowerEdge R220 II - Max hard disk size?