I don't think it's slot draw.
I can solve problem by moving the 6 pin to another source.
But it is weird that issue is caused by very fast spike that doesn't show in HW Monitor.
The reason I'm betting on slot (PCIe bus) draw is that unlike many PC mobos, Apple's Mac Pro mobos do
not have a
PEG connector to supplement power to the PCIe bus. This perfectly explains why the Pascal cards shut the machine down outright when another PCIe card is in use and when CUDA-Z ramps things up in Heavy Mode. Oddly enough, this actually contradicts my concerns regarding using SATA power to supplement the GPU power requirements because that's exactly what a PEG connector does on PC mobos. The main concern then really, is any device on the SATA cable in addition to the GPU's PCIe connector - any power draw spike (such as CUDA-Z) could potentially fry any device connected in the daisy chain on the same SATA cable, and since Apple uses a single SATA cable with dual headers, you see the problem.
The only (relatively) safe configuration for Pascal in a Mac Pro is muxing the two mini-PCIe 6-pin into a single 8-pin via the appropriate adapter and using the SATA cable for the remaining 6-pin connector on the GPU
and not using
any additional PCIe cards, as the MPs don't have a PEG connector to supplement bus power to the slots.
You can have both a power draw spike and a power "droop" at the same time, ironically. The spike in amperage can actually lower the voltage enough to trigger a fault and shut the machine down even if the total wattage is within spec. Apple's PCIe power path on both the slots
and its two 6-pin connectors is designed for steady loads, not spikey loads. Those mobo traces aren't designed for high demand loads like full-on shielded and insulated thick gauge cables are.
Bottom line is this: If the Pascal card has two 8-pin connectors, there is
no safe way to run it in a Mac Pro outside of using an external PSU such as one of those that goes into the optical bay and connects to a real plug through the PCIe slot exposure in the rear via a specially designed cable. And the only workable way to run any other Pascal GPU that has a mix of 6 and 8 pin connectors (one of each) is mini-PCIe 6-pin x2 -> PCIe 8-pin x1 adapter coupled with SATA -> 6-pin PCIe adapter for the remaining 6-pin connector, and you can forget about using any additional PCIe cards.
Mac Pro PSU can handle the spikes. But the motherboard cannot.