Are there any issues with having six disk drives in a Mac Pro?
Purely anecdotally - I've been running 4 HDs in Mac Pros since 2008 with no issues - but never tried 5.

Currently I've a 4,1 with 4 HDs &amp; a boot SSD in the optical bay. I still have the optical in the other bay.

I find that normally the CPU fans run a little fast at stock settings, so I use iStat Menus to lift the Intake fan speed from its nominal 600 to 1000 rpm &amp; that settles everything down.

Pure speculation, but that's because I'm running 2x3.46Ghz Westmere Xeons instead of the original Nehalems, so ymmv.

The only thing I'd beware of would be to give it enough room to breathe, plenty of exhaust space. I find my older 3,1 whilst perfectly capable of keeping its own temperatures down, really throws a lot of heat into the room, so I wouldn't ever put it in an enclosed space.

Potentially, with an HD in the optical bay, you might want to lift the Power Supply fan speed, as the HD will be pushing more heat than the opticals ever would &amp; I'm not sure the system would automatically compensate for that.