On Fri 21 January 2011 10:42:39 Goffredo Baroncelli wrote:

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Thanks Goffredo but as I say, I did this and it responds with 
 "ERROR: error adding the device '/dev/sdc'"
.. it doesn't give a clue.

In your email you wrote that before adding the device you format it.
Anyway I don't think that this is the problem.

Have you check which is reported in dmesg.

Absolutely no sign of a problem in dmesg or the logs.

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btrfs device scan

This must be done at every boot?  If so, where is recommended, in rc.local? 

Look at this thread, where I wrote anout a possible solution

http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg04709.html

Thanks.  I am suspecting scan is built into Debian.  Not sure what to look for though, with all the darned variables.

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2.6.32-5-amd64 with Debian patches.

From a btrfs point of view, it is a very old kernel. IIRC in this kernel
is not supported the snapshot removal. I suggest you to update the
kernel with a recent one.

I've looked for a newer kernel sources, but there isn't one with Debian patches.  I always compile my kernel.