I have read enough positive reports of the Startech Dual M.2 card (PEX8M2E2) working in MacPro 5,1s that I decided to give it a try in my mid-2010 MacPro 5,1. For the NVMe M.2 SSD, I found a good deal ($90!)on a 1TB ADATA XPG SX8200 Pro, installed that in the Startech card's M.2 #1 slot, and then installed the card in my MacPro's second 16x PCIe slot (#2).
The SX8200 Pro showed up ready to initialize as soon as I booted, so I formatted it in APFS without issue, and in System Report it's recognized in the NVMe Device Tree with x4 / 8.0 GT/s link width & speed - the same as what's reported for the Startech card in System Report's PCI card list.
Before attempting to clone my Mojave boot install from my WD Black HDD, I wanted to do some speed / stress tests. It didn't take long before I experienced the same kind of hard crash that
@kindkind reported last month, after about a minute and half into a speed test in Blackmagic (using the default 5GB stress test size). After my MacPro rebooted, I tried just copying some large files from my HDD to the SX8200 Pro and it locked up again after about 4GB had copied. I use iStat menus and set it up to show the SX8200 Pro's temp, so I know it never got over 33° C before the OS locks up.
Since
@kindkind said reformatting their 970 blade to Mac OS Extended and then back to APFS got it to start working, I gave that a try. Then I did the HDD to SX8200 Pro file copy test again, which was successful. But when I tried the Blackmagic test, my MacPro locked up in less than 30 seconds. It's frustrating because the read/write looked pretty good, at around 2500 MB/s read & 2200 MB/s write.
I know some people have speculated that the Startech PEX8M2E2 needs blades installed in both M.2 slots to work, but the specs on their website does not state this, and in fact claims "read/write speeds up to 3GB/s and 2.8GB/s
for single-drives or 2.4GB/s and 1.42GB/s (per drive) for 2," implying that using
one drive with the card will result in better speeds (unless they are talking about speeds when two drives are in
simultaneous use). Also, I have read a few specific reports from MacPro 5,1 (both mid-2010 and 2012 models) of single drive installations working great - even as boot drives.
Next, I tried switching the SX8200 Pro blade to Startech's 2nd M.2 slot, but I still got hard crashes. So I tried moving the Startech PEX8M2E2 card to my MacPro's PCIe slot 3 (4x). This got the SX8200 Pro fully working, but at a much lower speed, of course, at no higher than around 1500 MB/s for both read and write. I was able to clone my MacOS Mojave HDD boot volume, and the cloned OS booted just fine from the SX8200 Pro. I tested it for a week, using my Mac Pro for work (preparing art for large format printing in Adobe InDesign, Illustrator, and Photoshop), with zero issues. Based on this I decided the Startech PEX8M2E2 card I got must be working normally, and perhaps the issue could be related to using it with only one M.2 SSD blade.
I found someone selling a 2 year old 256GB Sabrent Rocket blade for $30, installed that, and then put the Startech PEX8M2E2 card back in my MacPro's second 16x slot. And, SUCCESS!!!
The SX8200 Pro is no WD Black SN-750 or Samsung 970 Pro, but for a $90 1TB blade I am very happy with the speeds: up to 2750 MB/s read & 2340 MB/s write.
The used Rocket, on the other hand, only clocked in as high as around 1500 MB/s read & 1050 MB/s write.
I think if there's a way to issue the ATA Secure Erase command to the Rocket, I might see better speeds. I just don't know if my non-flashed GPU will display when booted into Linux to run GParted or similar. Anyhow, for $30 I'm fine using it as a scratch disk.
The lesson learned here is that it seems the reports of the Startech Dual M.2 card (PEX8M2E2) needing blades installed in both NVMe slots in order to work on (some?) Mac Pro 5,1 models are true!