Checking other superblocks in BTRFS
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CITATION: http://www.funtoo.org/BTRFS_Fun
1) get the latest version of btrfs-tools
See here: HERE
2) test another superblcok:
# btrfsck --s 2 /dev/sda3
3) restore the default superblock (#0) with the better copy you found in location 2
# btrfs-super-select -s 2 /dev/sda3