Store bought NAS doesn't run open source software.
Some of them can be coaxed into running normal distros. Buffalo Linkstations, for example.
Any recommendations on PCIe adapters which work with both slots on the X220, since only one of them is ExpressCard?
I just solder directly to the motherboard, there are 3 usable PCIEx1 interfaces on x220, one at the missing USB 3.0 controller footprint, second one mpcie, third ExpressCard. You could also desolder memory card controller for fourth pcie lane. Then you have 3x SATA controllers, one HDD, one on mpcie socket, and third on the Dock connector. You also get over 10 individual USB 2.0 channels. There was a time before Sandy Bridge you could configure pcie bifurcation and freely join separate lanes, but I dont think anyone figured out how to do it on x220.
Have you considered writing a forum/blog post or Tell HN on your X220 NAS mods?
Its a bare pcb in old 16 port switch case with couple of drives stuffed into the bottom of my closet. I have another x230 pcb stuck into 4337 dock (got those for ~$3) just so I can have a nice power button.