Comment # 12 on bug 980065 from Stefan Hundhammer

So, this is an unsupported scenario, and AFAIK it's not trivial to support this
with the old libstorage. The upcoming libstorage-ng might support this on the
long run, but we have a lot of other things to get in there first.

In the meantime, we can only advise you to use LVM as an intermediate step for
such a scenario; or use a larger partition for Btrfs in the first place. You
might have to reshuffle things a bit, but this is nothing that can't be done
with booting from a USB stick with a live file system on it and copying around
files for a while; I'd copy the files from that Btrfs on /dev/sda2 and
/dev/sda4 (both on the SSD) to free space on /dev/sdb? (or use a USB disk if
there is not enough free space on any /dev/sb? partition), /home on /dev/sda3
to another directory, then repartition the SSD (/dev/sda) to have it less
fragmented, then copy things back.

When you do that manually, you can even keep the old UUD
(c5a20327-9e28-48d5-8321-96a35f02df84) of that Btrfs; mkfs.btrfs has a --uuid
command line option for that case.

Sorry that there is no simple, automated solution to this, but AFAICS this is
the best we can do for now.


Bottom line: Unsupported scenario, so it's a mixture of INVALID and WONTFIX. 
But there is a feasible manual solution (see above).