http://moq7aejnf4xk5k2bkaltli3ftkhusy2mbrd3pj23nrca343ku2mgk4yd.onion/stuff/article/rpi_root_on_external_drive?lang=en
If you have a system previously installed on a microSD card, then move the files to a new root partition using cp -a /path/to/sdcard/root/* /path/to/new/root . -a stands for archive, which recursively copies everything preserving all the attributes and links.