I've been joking about this project with a colleague while we were passing USB sticks back and forth to get some piece of software running on a site without internet.
With the Pi Zero (cheap, small form factor, enough processing power, USB, SD card, GPIO) and some WiFi adapter to connect to the remaining IOs (ESP8266 to start out with) it is possible to have the devices connect to each other (by having one set up an access point or using WiFi Direct), forming The Borg Drive that automatically synchronizes drives mounted on the PIs' SD cards while presenting as a generic Mass Storage Device to the host PCs.
It'll be my first "real" PI project so help is greatly appreciated and even if something similar already exists this can still serve as a reasonable learning experience.
Connectors and Accessories / Telecom and Datacom (Modular) Connectors
High Efficiency Buck-Boost Regulator with 4.5A Switches in a QFN Package
WE-MAPI SMD Shielded Power Inductor 1µH 7.2A 4x4x2mm³
Power Management ICs / Power Supply Support
>> The BCM283x SoCs at the heart of every Pi has two MMC controllers. One
of these connects to the SD card slot that the Pi boots and runs its OS
from. The other has largely been ignored, but comes out on the HAT connector
as alternate functions on GPIOs 22-27. Recent kernel and boot blob
changes have enabled these pins to be mapped to the MMC controller that
also supports SDIO devices.
>> The SD cards are primaraly a SPI device, as such you technically could
parallel anouther device as long as you use a seperate chip select line
ut this would then require special driver to know what to use for it.
Sometimes it is the hardest to find definitive statements about obvious things. ajlitt has done an excellent job documenting all there is to know about the MMC controller and the secondary SDIO interface. The facts have changed over the past time so I initially found some contradictions.
Looking forward to Jacksons PCBs and ESP 12F modules from abroad.