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Did you take a look at the README? https://github.com/rancher/k3os#automated-installation

yeah, but it doesn't really hint if

url

is allowed to be a filepath on another partition

kind of a chicken-and-egg scenario here. iirc the url can be a regular filesystem path but that assumes that something has mounted the locations where such would exist (which you need config in place to do for you). so, unless it is in the initrd (or the root disk) you're gonna have a hard time referencing such.