Sonnet M.2 4×4 is a full-length, full-height PCIe card that can host up to four M.2 SSDs, which can be combined for extremely high performance. The card offers an X16 interface to the PCIe 3.0 bus with x4 bandwidth for each M.2 2280 NVMe SSD, promising extremely high performance via software RAID configurations in appropriate host systems, such as the 2019 Mac Pro. A single (non-RAID) SSD can function as a boot drive. The card is compatible with Thunderbolt PCIe expansion chassis, though maximum performance is restricted by Thunderbolt’s bandwidth limits, and similar restrictions apply to PCIe 2 slots.
Sonnet M.2 4×4 [Amazon link] is due to ship Jan. 2, 2020, list-priced at $399.99 (sans SSDs) for host computers with appropriate PCIe slots running macOS 10.13 and later, Linux Kernel 5 and later, or Windows 10.
OWC Accelsior 4M2 is a PCIe card from Other World Computing Inc. (macsales.com) that hosts four M.2 NVMe SSDs. Each SSD socket offers full x4 speed for maximum performance, and the full-height, half-length card handles PCI 3.0 at x8 speed in a 2019 Mac Pro or other computers (including slower PCIe 2.0 systems, such as classic Mac Pros).
You can boot off a single M.2 SSD, while the bundled SoftRAID app can create RAID 0, 1, 4, 5, 1+0 (10) volumes (sans support for FileVault encryption) for speeds “over 6,000 MB/s” (RAID-0, SoftRAID, two cards) with up to 8TB per card. Other features include “silent cooling via a finned heat sink cover”, an ASMedia ASM2824 controller, 380TBW endurance, and Trim support (OS-dependent).
OWC Accelsior 4M2 is priced from $479.99 with 1TB (Toshiba TLC flash) to $1,599.99 with 8 TB. (Apple charges $2,600 for 8TB storage in a 2019 Mac Pro.)
HighPoint SSD7101A-1 is a PCIe card from HighPoint Technologies Inc. that hosts four NVMe SSDs for ultra-fast storage. Features include PCIe 3.0 x16 bus bandwidth and x4 bandwidth for each of the NVMe M.2 SSD slots, plus a built-in RAID controller (for RAID 0, 1, 5 and 10) with management software and SMART support and the ability to pair two of the cards for astonishing speeds. (See resources section for more details.)
HighPoint SSD7101A-1, for macOS 10.13 and later, Windows and Linux, is sold via resellers, including Amazon ($387.99 when checked).
HighPoint SSD7103 is a bootable version ($449 when checked).
“The latest generation of high-performance CPU’s and motherboards can provide the PCIe 3.0 lanes required to power dual PCIe 3.0 x16 NVMe RAID controllers. Ultra-HD video projects, such as those working with raw 8K footage, require transfer performance exceeding 17,900 MB/s. Media professionals working with such hardware can now link two SSD7101A-1 NVMe RAID controllers to function as a single array, and configure RAID arrays of up to eight M.2 NVMe SSD’s to deliver over 25,000 MB/s of transfer performance!”