On Mon, Mar 14, 2016 at 10:59 PM, Chris Murphy [off-list ref] wrote:

I'm a little mystified how btrfs check reports a problem with the
superblock, and yet this filesystem can still be mounted and used? If
it mounts rw then resize is possible but why would it be wrong in the
first place?

Yes you are right. I just focussed on the 'Superblock bytenr is larger
than device size' part.
btrfs check  will actually output that text on a mounted filesystem anyhow.
The other question is: What is mounted on /media/tempdisk/  ?

At least I think a check of the current 200GiB fs is needed. As it is
a rootfs and encrypted, some work is needed to make that happen.