You could look for a controller board with a chip on it that
drives a SATA M.2. But this would use one of your PCIe slots,
and that might be all that desirable (your slots are
probably all used by now).
https://www.amazon.ca/StarTech-com-M-2-SATA-Controller-Card/dp/B017IM54GM
*******
https://en.wikipedia.org/wiki/M.2
"The M.2 specification provides up to four PCI Express lanes
and one logical SATA 3.0 (6 Gbit/s) port, and exposes them
through the same connector so both PCI Express and SATA storage
devices may exist in the form of M.2 modules."
*******
There really isn't a good reason to buy a SATA version of
an M.2. That would only run at 450-500MB/sec or so. Whereas
the PCIe x4 version of M.2 does around 2GB to 2.5GB/sec in
some cases (it can't do 4GB/sec because of the limited
size buffers on the Intel end for one thing).
With modern motherboards, it pays to find the PCH datasheet
and the section on "FLEXIO". As that will tell you what
enabling one thing will disable some other thing. However,
you still need to understand which facility of the FLEXIO
is wired to which slot or connector. Having this understanding,
compensates for motherboard manuals lacking in the proper
details. It's possible Anandtech has had copies of the
FLEXIO picture in some of their reviews.
I think it "suspicious", that the M.2 with SATA capability,
backs right up to one of the right-angle SATA stacks. That's
probably a coincidence, as the switching of the function should
be in the PCH. You can't drive two connectors
off one port, and unlike expansion cards, the motherboard
is unlikely to use jumper plugs to control which
direction the port wiring goes to (left or right). Since it's
likely the PCIe wiring and the SATA wiring share some diff pairs,
the PCH is likely routing a particular port, to a
(limited) set of SATA logic block . Likely arranged
as two identical four port controllers or something.
The FLEXIO likely needs a kind of crossbar, so wire pairs
on the PCH can be routed to the correct internal logic blocks.
I used to absolutely *hate* the FLEXIO concept when I was
doing board design. Intel isn't the first to use this,
and the concept has been used elsewhere. Some stupid chips
have five functions bound to one pin, and you sit there at
your desk for hours, juggling the damn things and trying
to get the best usage out of the interfaces. The boss is
putting on the pressure to produce, while you're whiling
away the hours playing "crossword puzzle" with some stupid
chip docs.
In the Asus manual, I can see that PCIeX4_2 shares with
SATA5678, but no mention is made of the M.2 needing to
rip off a port on one of the controllers.
And you have a 28-lane CPU, with the restrictions that gives.
I'd probably start by verifying that moving the x4 boot
drive over to the other M.2 slot works without disabling
anything else. And then look into exchanging (or paying
a restocking fee) for the M.2 SATA flavored device. The
thing is, even if you used a SATA adapter card in an
attempt to get some usage from the SATA M.2 somehow, that
uses a PCIE slot, and maybe that would have adverse consequences
on yet some other sharing inside the PC.
Somehow you're going to have to figure out how the
FLEXIO works. It's possible that M.2 slot fully
terminates on the PCH, and it's the PCH crossbar
that moves the internal SATA block from using
the right-angle conventional SATA connection, over
to the M.2 slot.
Paul