This is an adapter board to put two SSDs in an M.2 slot that only accepts one. I've designed this for myself, but I'm sure it'll be useful to many people like me who only have one M.2 slot available and no other storage expansion options in their daily driver laptop!

You need to solder a SATA link to this adapter (7 wires on both ends, 14 connections total), and it will help you with all the other implications of having an M.2 slot - power, control signals and the SSD's physical placement.

If you have a PCI-E/SATA hybrid M.2 slot, oftentimes, there's a Pericom-made (or similar) mux on your motherboard (sometimes, on the opposite side of the board) that switches either PCI-E or SATA to your M.2 slot, and it's likely that you can tap SATA from that while you have an NVMe SSD inserted. If that's not an option, you can use the 2.5 HDD SATA port you might have available and unused, or add a USB-SATA adapter inside.

There are enough 3.3V pins in an M-key slot for two SSDs, and the M.2 slot is typically connected to a large 3.3V power rail that powers everything in a laptop with a good margin to spare. If you want to be absolutely certain it works out for you power-wise, it's good to check the schematics for your particular laptop. This board has no decoupling capacitors, but they don't seem to be needed - that said, if you encounter problems, do let me know, and let's see what we can do.

This board can be shortened to 2230 or 2242 size if you need to fit it into a different socket. I hope you can manage the mechanical implications well enough =) The resulting 3-board sandwich will be thicker than the usual SSD, but I've taken care with socket heights so that it's as thin as it's viable for it to be!

This is a fully open-source product! Use the "Design files" link below to get the KiCad files for your own research.