How do you make a card that doesn't work in Windows? If a PC has UEFI and a UEFI NVMe driver, then it should just work. It should work in a Mac if it has new enough UEFI to boot Windows.
I'm just going by what it says in the OP (see the red text):
Sonnet M.2 4x4 PCIe Card FUS-SSD-4X4-E3: Supports four single sided 80mm M.2 blades. Uses x16 PLX PEX8747 switch. Heatsink with a fan over the blades and PCIe switch. Needs a GPU with pre-boot configuration support. Almost to big to fit MP5,1 length wise. Do not buy it if you need to boot Windows from the blades or will upgrade to a PC later; this card can't boot Windows. Sonnet has a support document about what major blades are single and double sided:
https://sonnettech.com/support/downloads/manuals/M2_compatibility.pdf[/quote]
Without a boot screen, you need to be able to set the boot in macOS (use the bless command if Startup Disk preferences panel doesn't have the option). In Windows, the Boot Camp control panel may be useable, otherwise you can use the EasyUEFI app. I think there's a thread somewhere describing how to use Open Core to boot Windows "safely" using UEFI on old Mac. I think there's a thread somewhere describing how to get boot screen with rEFInd or Open Core (I'm not sure which or where - one can chain boot to the other). I haven't tried booting Windows with UEFI since my MacPro3,1 is too old for that - only Linux and macOS will boot properly using EFI on the Mac Pro3,1. I haven't tried getting a boot screen with a non-Mac GPU either.
I'm using a 2009 Mac Pro. It came with a 4,1 logic board but I upgraded the firmware to 5,1. But thanks for the info, I'll try it out when I get my card.
It doesn't have a PCIe switch so you'll only get up to ~ 1500 MB/s. You'll need a cable to connect a SATA NVMe (but you don't have a SATA M.2, so nevermind). They come in various lengths. Some with one or two power connectors:
P22PM-15PF-7P-6IN SATA 22 Pin Male to 7 Pin SATA Cable with 15 Pin SATA Female Power Cable
P22PM-15PF-7P-12IN SATA 22 Pin Male to SATA 7 Pin and 15 Pin Female
P22PM-15PF-7P-18IN SATA 22 Pin Male to SATA 7 Pin and 15 Pin Female
P22PM-P8-S18IN SATA 22 Pin Male to SATA 7 Pin and 15 Pin Female - 18 and 8 Inches
P22PM-15PF-7P-1M SATA 22 Pin Male to SATA 7 Pin and 15 Pin Female 1 Meter
P22PM-15-7-S3 SATA 22 Pin Male to SATA III 7 Pin and 15 Pin SATA Female - 12 Inches
P22PM-2X15PF-7P-2IN SATA 22 Pin Male to SATA 7 Pin and 2 X 15 Pin SATA Female Connectors - 2"
P22PM-2X15PF-7P-6IN SATA 22 Pin Male to SATA 7 Pin and 2 X 15 Pin SATA Female Connectors - 6"
Thanks for this. It turns out my order didn't go through properly so it's not coming, I'm reconsidering buying this.
It was kind of a joke because 400 MB/s (4 Gbps) max would be a waste for an NVMe)
External enclosure should be fine for EFI boot (not legacy BIOS boot which is what you usually use to boot Windows on old Macs - where Windows disk needs to be connected to one of the internal drive bays). NVMe requires PCIe.
External SATA (6 Gbps) might be the least expensive way.
No one has made a USB 3.x booter (10 Gbps) for classic Mac Pro yet.
Thunderbolt (22 Gbps PCIe) doesn't work well enough for booting a classic Mac Pro but I guess it's not impossible. It might take as much work as creating a USB 3.x booter.
External PCIe is super expensive (there's a subset of products at B&H Photo - each manufacturer has their own website - I have the Dynapower USA Netstor NA255A 16 lane host (64 Gbps) with PCIe 3.0 switch connected to my MacPro3,1), or you can make your own with a cheap single lane riser (4 Gbps) like the one I linked or you can get 4 lane (16 Gbps) cables at ADT-Link. Only a product with a PCIe 3.0 switch with at least a 8 lane host adapter (32 Gbps) will give full performance from a PCIe 3.0 NVMe device (31.5 Gbps). There exist products with PCIe 4.0 switches - but I haven't seen anyone try that with an old Mac Pro yet - you would need an x16 host adapter (64 Gbps) to get full performance from a PCIe 4.0 NVMe device (63 Gbps) connected to a classic Mac Pro.
(Host bandwidth numbers assume Mac Pro PCIe 2.0 slot or PCIe 2.0 switch)
Thanks. I might just bite the bullet and buy one of the more expensive four slot cards. I'm leaning towards this one because it says it can boot windows:
HighPoint SSD7101A-1: Highpoint 7101A - PCIe 3.0 SSD performance for the cMP. Supports four double side M.2 blades, up to 110mm. Uses PLX PEX8747 switch. Heatsink with a fan over the blades and PCIe switch. This card can boot Windows when using NVMe blades and UEFI install.
Before I buy it I'd like to hear your thoughts though, just in case it won't work in my system. Again I'm using 4 single sided 500 GB Samsung 970 Plus blades in a 2009 Mac Pro with a 5,1 logic board and Mojave installed. My bottom 16x slot is occupied by a Radeon Saphire Pulse graphics card and I don't have a boot screen.