adafruit-pi-externalroot-helper script is mostly an implementation of the process outlined in paulv's HOWTO: Move the filesystem to a USB stick on the Raspberry Pi forums, with additional help from the following sources:
- The Arch Linux wiki, GUID Partition Table for what a GPT is and some
- GUID Partition Table, Wikipedia.
- Booting from an external USB drive, from the Raspberry Pi StackExchange.
- Booting a Raspberry Pi reliably from USB in the presence of multiple USB drives, by Stefan Krastanov.
- Speed up your Pi by booting to a USB flash drive, by Sam Hobbs.
This comment on
init/do_mounts.cin the Linux source tree.
While the steps here are fairly straightforward once you put them together, this can be a finicky process. If you run into any problems, please consider filing an issue on the GitHub project.