You might want to consider a PCI-express NVME adapter. This one worked great for my bro before he upgraded his PC, and there's currently an amazon warehouse deal for one at $4.67:
https://smile.amazon.com/gp/product/B01I5VABFY/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
You won't be able to use it as a boot drive on most older motherboards, but it works great for storage (runs at advertised speeds).
Can check with AIDA64 (Computer -> Summary -> Motherboard). Should not conflict with SATA at all. Keep in mind you could run a M.2-to-PCIe adapter in your second GPU slot (PCIEx16_2) to achieve 4x speeds (see pg. 1-11). Although I don't think x4 is necessary.
I recently upgraded a Dell T5810 with this PCI adapter. The NVME is BIOS-recognized and bootable. No need to involve other storage devices.
Put those prices in Checkmate.
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Quite true. Should be fine in a M.2-to-PCIe adapter or USB enclosure (albeit limited to 10 Gbps in the latter case, realistically), or even in a 2x M.2 socket as it will still get those all-important writes if it's used effectively.