I'd like to mirror my existing root (and only) partition on an SSD to another disk. It should be a sort of RAID-1, just asymmetric\*. I know there's the option `mdadm --write-behind`, which should do it.

But I have no idea if it is possible with preserving the context of the existing partition. I imagine it like

* create the "slave" partition
* setup the RAID telling it that the slave partition is not initialized
* let it initialize it by cloning the master partition

but I'm probably too optimistic, aren't I?

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\* All reads should access the first disk and writes should be considered finished when the first disk is written.