I too would like to report success upgrading my MBWEII (White Light), but from 2TB to 6TB. Like bigor_2 above, I independently guessed that this was possible, and managed to get through steps 0 to 9 intuitively; however, I was also somewhat dismayed when my RAID volume size was showing up as 997GB. Therefore, a big thanks to DThought for his instructions from steps 10 to 15 to fix this, because as I write my MBWEII is showing a RAID volume of 2.73TB and is 38.3% through its resync.
Just a couple of points that might help anyone else going through this in the future:
a. I would be considered what used to be called a computer "power user", but certainly not a developer/programmer…and until the other day I had never touched UNIX in any way; especially not the command line. That said, I did start using computers in the days of DOS, so do have a basic understanding of how command lines work (which may not be that case if you have only ever used a GUI). In short, it wasn't hard, but you do need to be exacting in your approach.
b. I am now a Mac user (for about five months) after many years of Windows use, and simply used the OS X Terminal to login as the root user for this project. I have found this exercise very informative, because a knowledge of UNIX commands has come in handy for moving data more quickly around my MBWEII and MB Live Duo (MBLD), as well as exploring the underlying UNIX foundation of my MacBook Air.
c. Using the steps 0 to 9, I managed to preserve everything that was already on my drives (82% of space used in Mirror mode of the previous 2TB MBWEII).
d. I did not move drive B to the drive A slot as suggested at steps 6 to 8, and it still all worked out fine.
e. I was simply copying and pasting the commands from here into the OS X Terminal. However, for some reason the command at step 14 arrived as "mdadm -grow -size=max /dev/md2" when pasted into Terminal. It took a bit of research to discover that the command syntax should have been "mdadm grow —size=max /dev/md2". I think Safari might be converting "" (that is, two hyphens) into "—" (that is, an en- or em-dash), which is then pasted as a "-" (that is, a single hyphen) into Terminal.
f. Like Darr247 above, I had to use the command "xfs_growfs /dev/md2" at step 15, as DThought's suggested "xfs_grow /dev/md2" did not work either.
I got my two WD 3TB Caviar Green HDDs (WDC WD30EZRX-00M) from a US ebay user for a little over US$140 each delivered to Australia. They were "lightly" used, but well below the best price I could get in Australia (always very expensive here) or Asia. Anyway, I knew this was an experiment from the start, and figured that I could re-sell them for only a small loss if it didn't work out. As it is, I will now sell my two 1TB HDDs that are now surplus to my requirements to defray the costs involved…I just need to figure out how to wipe them.
Finally, everything seems to be be working out fine. However, I was previously using Panic's Transmit 4 to move data around using SFTP (at least I think I was, although this might have been on my MBLD), but for some reason, now it will not allow me to move between directories on the MBWEII. I can still move data using Finder and Transmit's Finder equivalent (but very slowly in both cases) and using the Terminal command line (much faster), so I do not know if that has been caused by going through this exercise. That reported, I have also been having some funny directory and file ownership and permission issues with both the MBWEII and MBLD as I have been playing around over the last week, so this could just be an extension of those problems.