Pinebook Pro 'xtreme'

[xyzzy]

First off, I would not want a NVMe -> eMMC adapter. That's not what I suggested. A simpler PCIe -> eMMC interface, (or dual eMMC), would be much more appropriate. Then deal with the difference in software. If it had to be bootable, then an additional driver for U-Boot would be needed.

What simpler interface would that be?  You can't use "eMMC over PCI-e".  It just doesn't work that way.

It's like a USB reader for SD cards.  They don't appear as "sd cards".  They look like a USB Mass Storage device to the computer, because that's a protocol that can work over USB.

So if you want a PCI-e interface to eMMC, you need an protocol that can work over PCIe.  Like NVMe.  Or you could look like a SATA controller I suppose, but NVMe makes more sense in every way.  You could invent your own protocol and then you get to write drivers for it.  But there's nothing wrong with NVMe.  It's designed for this.

But either way, this adds a translation to and from a HS200 (or other MMC) bus in the middle and splits the PCI-e interface and FTL into different pieces of silicon.  It's everything that's in a normal PCI-e SSD plus an additional interface plus it's less efficiently designed in two chips.

So it's not going to use less power.

If you really want to add storage and are dead set against a normal PCI-e SSD, then try this, Ableconn M2SD139PM M.2 B-M Key Module with Micro SD Socket.  It looks like the chip has a Linux driver.

Or just want for SD cards with the new SD7.0 standard to appear.  They will have two new pins that support a PCI-e interface.  Yes, SD cards will become PCI-e cards.  And what protocol do they use over this PCI-e interface?  NVMe.  Like I said at the start, it makes the most sense.