I have fixed the problem. It was purely an air flow issue and a few small pieces of foam pipe insulation fixed it.
What did I do, well I evaluated the air flow which meant I removed the front cover to access the fans. While I was examining the fans I noticed that I could feel a lot of air coming out the sides which actually should have been an obvious thing to see originally but I see it clearly now. So the majority of the air flow was going around the drives (path of least resistance), not through them. I immediately saw the temps lower to two drives at 29C and two drives at 30C. Major improvement. Next I dropped the fan voltage back to 7VDC and started a scrub and after running this for just over an hour my temps are 31C to 33C.
The long term fix will be to take a sheet of galvanized metal (typically used in HVAC systems) and cut it to shape and then screw it into place using four screws (or I have this double sided automotive tape, sticks like epoxy) ensuring most of the air flow is directed through the hard drives. Doing this will allow me to remove the foam tubing and give it a much cleaner look. I'll take photos of the case modification.
Photos:
The first photo is of the left side of the case, notice the gap between the front of the case where the fans mount and the side of the hard drive cage. This is about a 1" gap and allows air to flow around the drives, not through the air flow holes in the side of the drive cage.
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This is the backside of the drive cage, you can also see the gap, note the silver standoff, this is the section I'm talking about.
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And here is a front view of the case.
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Here is the item I'm inserting...
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And here is what it looks like when I have installed these two pieces of foam to direct the air flow.
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Note that the holes blocked in the front are not important, it's the fact that the air flow is blocked from escaping out the gap between the rear of the fans and the hard drive cage. I could get a single large 200mm fan in and air flow would use these presently blocked holes.