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Intel Customer Support Technician
Under Contract to Intel Corporation
The Intel RST feature can only be used with the chipset's SATA ports. I believe that, in Desktop, this is limited to six SATA ports (though Server chipsets may support more; I am unsure). While it is true that you could, using two (or three) M.2 NVMe SSDs, have a total of eight (or nine) SSDs connected to the chipset, you cannot typically mix interfaces. That is, you can create a RAID array using two (or three) M.2 NVMe SSDs and you can create a RAID array with up to six SATA SSDs, but you cannot create a RAID array using both M.2 NVMe SSDs and SATA SSDs. The one exception to this rule is that an Intel Optane Memory module, plugged into a M.2 NVMe interface, can be used, through caching, to accelerate the performance of a single SATA SSD/HDD/SSHD.
Hope this helps,