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#1
Is this an internal or an external USB connected 6 TB hard drive. If it is an external USB powered drive it's likely that it is drawing too much power from the USB port. You may need to buy a self powered hard drive caddy, that supplies it's own power & not use the USB power supply.
It's internal. The PC with MB + PS were bought with the intention of many HDDs. I just switched power plugs between HDDs so far so good. Let's see later today how the copying between HDDs goes.