Comment 56 for bug 599255

Oh dear.

It seems this version of dmraid won't handle gpt!
So you may be a little stuck with using partitions of that size.

There is a thread here where someone has made a patch: http://ubuntuforums.org/showthread.php?t=1369224
I will have a look at it later to see if I can incorporate it into my dmraid in my ppa!