Why did two of my SATA drives vanish after adding a second M.2?

Question

I added a second M.2 SSD to my B650M Pro RS purely for game storage. First boot after that, two of my SATA drives were just gone from the BIOS. Figured it was a cable, so I swapped both of them out, no change. On a hunch I pulled the new M.2 back out and both SATA drives showed up again immediately. So it clearly has something to do with that slot. Is this a sign the board is failing, or is this actually normal behavior I didn't know about?

tangled_cables · Accepted Answer

Lost two SATA ports like this on an older board, took me completely by surprise back then. Secondary M.2 slots share chipset lanes, so populating one can drop SATA ports or narrow another slot. The mapping is model specific, so your manual's lane-sharing table is the authority. On the Strix X870E-E, filling M.2_2 and M.2_3 drops the main slot to x8.

pixelpusher99 · Answer

Had almost the exact same thing on my B850 Pro RS... lost a SATA port the second I filled the bottom M.2. Took me a while to realize the two were even connected.

smoothframes · Answer

this is the one upside of ITX. my b850i only has the single M.2 slot so theres nothing to steal from