Is there a 'friction-free' way of converting a RAID1 to a RAID5? What's the best way to convert a Linux software (that is, `mdadm`) RAID1 array to a RAID5 array? If the procedure is different for the cases of 3-disk RAID5 vs. 4-disk RAID5, how is it different?

Before doing anything of this sort _back up your data to separate media_ and _verify the backup_ via `sha1sum`.

The general procedure from there would look like:

* Add the third drive to your system.
* Create a degraded RAID5 out of the new drive and the one freed from the RAID1.
* Copy the data over to the RAID5 volume.
* Add the RAID1 disk to the RAID5 volume, and give it plenty of time to synchronize itself properly.
* Verify that the data on the new volume matches the backup.



Also refer to kernel.org's Wiki page on mdadm, and, for the opposite direction (RAID6 to RAID1, at least), there's a question on serverfault.com, where one of the answer also mentions a blog post about mdadm level changes.