I'm wondering about the added heat from that internal Pegasus Hard Drive.
The MPX provisioned drive? The R4i is sitting in the same slot meant to cool a VEGA II Duo at 400+ watts. Four hard drives and a raid electronics card aren't going to tax that kind of air flow cooling at all. Unless the R4i screws up the fan speed feedback adjustments for the MPX bay 2 fan, that shouldn't be a problem.
The non slot , dual drive adapter? The J2i isn't going to product much heat at all. It is the hot air that is blow
at it that is a possible issue. It is basically sucking up output from the CPU heatsink. The heat is spread over a larger area, but will get hotter than ambient air. Hot enough to shortly fry the drive? Probably not with relatively normal temps outside the Mac Pro. Hot enough to impact long term longevity of the drive? Pretty good chance it will ( and longer and hotter run the CPU then higher that chance gets.).
if they are drives with valuable , archival data on them, I'd put them outside. They will be more affordable solutions.
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There APPEARS to be SATA ports inside the machine, but no info if they are directly accessible via standard SATA cable > drive, or how you would power if additional is needed.
It isn't really appears. The J2i has to hook up to something both SATA data and power. The connectors for two drives and a power cable are there. There is also a USB Type A connector internal also. ( use for a iLok or security FOB. Or could probably even be used for a small alternative boot drive. ).
The Pegasus products are closest thing to internal drive bays like MP5,1. Probably overkill and over budget for your need.
There really isn't much to the J2i
https://www.promise.com/us/Promotion/PegasusStorage
it is a bracket that holds two 3.5" drives. That's about it. Promise is getting an early 'jump' here because they have had access to the Mac Pro. But once the physical attachment notch dimensions get recorded in an open setting there are going to be 3rd parties machining brackets to put in there. The question is has Apple let Sonnet or OWC or some other vendor have a Mac Pro demo box for last couple of months. Promise will probably have a "first mover" window there for a couple of months, but it won't last long. ( April-May 2020 someone will have something.... even it is some Chinese chop shop knock off. ) .