On Sat, 8 Jun 2013 20:18:10 +0200
Ramon Hofer [off-list ref] wrote:

My home server with a linear raid (md0) containing three raid5 (md1,
md2, md3) is still working wonderfully. Thanks again Stan!

Now I'm planning to add a fourth raid5 to the linear array.

You should have used LVM instead of md linear to join your RAIDs into a single
block device. LVM allows placing a single logical volume over physical extents
placed in all sorts of non-contiguous, fragmented, overlapping fashions. You
could also transparently migrate portions of those physical extents between
physical arrays without even having to unmount the filesystem that you have on
the LV.

-- 
With respect,
Roman