The manual for one of Asus's motherboards based on the B350 chipset states that a PCIe x4 M.2 drive can "share" bandwidth with drives on SATA ports 5 and 6. Gigabyte, however, says that the installation of an x4 PCIe drive precludes use of SATA ports 5 and 6. What's the truth here?
a. B350 can really support six SATA drives and an M.2 drive at the same time
b. B350 can only support four SATA drives with an M.2 drive installed
c. This depends on the motherboard and both Asus and Gigabyte might be telling the truth.
Thanks for any clarification. If Asus is mis-stating this in their manual I intend to hold their feet to the flame for falsely coercing a purchase that I otherwise would not have made
(By the way, I checked AMD's own specs on the B350 chipset and they are cryptic to say the least. They seem to agree with neither Asus's nor Gigabytes claims, which is adding to the confusion (and frustration).)
Here are ASUS' diagrams for easier understanding. The chipset (B350 in your case) supports 4 SATA ports. The onboard components of the Ryzen processor support either a single NVMe SSD OR two additional SATA ports plus an NVMe x2 slot. In ASUS' case it appears they are taking this route, hence the "sharing bandwidth" statement, whereas Gigabyte will just disable the 2 SATA ports if an NVMe drive is used or the NVMe slot if the SATA ports are populated. Cutting the speed to x2 really obliterates any reason for using NVMe anyway, which is why Gigabyte may be taking that route on these more budget oriented systems. Ports added by a controller card are, of course, not limited.
Experimentally, it appears to me that Asus actually handles it exactly as Gigabyte does. When an x4 drive is in the M.2 slot, SATA ports 5-6 are disabled. I agree this is preferable to downgrading an x4 drive to x2, but in either case, their manual is mis-stating the facts. The manual claims that an x4 drive can share with drives on ports 5/6. They don't explain exactly what "sharing" means, but they imply that it's a "graceful" sharing in which the SATA drive just temporarily eats only as much bandwidth as necessary from the M.2 drive. Certainly that is not the case.