Computer Does Not Boot With PCIe Card in x16 Slot - NI
On ZBOX Mini PCs equipped with an NVMe/PCIe SSD slot, current ZBOX BIOS' lists the M.2 NVMe/PCIe SSD separate from the standard SATA storage devices (SATA/Storage submenu).
How Many M.2 Slots Do Motherboards Have? - PC Guide 101
asus.com/au/Motherboards/PRIME-X370-PRO/specif says how many m.2 slots it has. Just becareful about the m.2 SSD that you buy (SATA vs PCIe) as the spec page is very specific on which CPU can support SATA only or SATA/PCIe.
Mainboard MSI Z370-A PRO (LGA1151v2-CoffeeLake)
* M.2 and SATA ports maximum support 1x M.2 PCIe SSD + 1x M.2 SATA SSD + 5x SATA HDDs. Please refer the manual for M.2 slots with examples of various ...