IF that's what you want, that's a fairly ingenious solution. If you look at https://macstadium.com/mac-pro https://macstadium.com/mac-pro you'd think you can ju

IF that's what you want, that's a fairly ingenious solution. If you look at https://macstadium.com/mac-pro https://macstadium.com/mac-pro you'd think you can just build the racks with the Mac Pro opening back and front. But it's 6.6" inches wide so you can only have two, three would be more than 19" (and you can't put 19" of equipment inside a rack). But this way you can squeeze four in the same space and since the cylinder is 9.9" high if you'd need you could squeeze in quite a few external hard disks as well although that would require using some fans to help moving air. You perhaps have 6-8 inches free space, one 3.5" HDD is 5.75 inch high so you could stand it on the shorter 4" edge and put in 6 taking 1" from the 19" and probably have two banks of this to arrive to 3 disks per Mac Pro and leave 4-6 inches still for moving air. It might not be impossible to squeeze in 6 disks per Mac Pro but cooling would need to be very impressive for that.