I'm experiencing similar problems with 4 x 8TB drives.  The back of the tower is around 12 cm from a wall.  Pulling the case another 10+ cm away SEEMED to improve cooling.  Theoretically that doesn't make sense since air is only exhausted out the back. I even wondered if maybe the hard-drive fan was installed backwards (it wasn't).
Because this solution is unintuitive, I'm checking if it was just a coincidence and maybe there is some other factor I haven't considered.  I'd appreciate hearing from anyone else who's dealt with this issue.

@Petro606​ yes indeed, the HDD cooling situation wasn't very well thought out on these workstations. I have 4 large capacity HDD's in the bottom four bays and 4 SSD's in the upgrade kit above, so what I ended up doing is taking two 120mm 4 pin PWM case fans ( both 2000 rpm ) that I pulled from and old computer and got a couple of USB to 12 volt 4 pin converters ( the 12 volt conversion is important! ) and mounted them on the outside of the front grill ( using twist ties of all things ) directly in front of each bay. I suppose you could simply purchase some regular USB fans, but the ones I had in my old PC are Noctua PWM fans, so they really push a lot of air and it was actually cheaper for me to buy the USB to 12 v converters and the fans are obviously of superior quality. Anyway, facing inward, they are pushing air in directly into both bays and it's working out BEAUTIFULLY! My hottest drives are my WD Black 8TBs which were constantly running around 45-50c and as soon as I mounted the fans it immediately dropped to around 35c ( 30 if it's cold outside ). My other HDD's are HGST 16TB and 18 TB which tend to run cooler anyway but they hover around 30 pretty much all the time ( 28 or so when it's cold out ). All of my SSD's are running at, or below 30c. If you're having a heat issue as I was, I highly recommend this as a solution.