Mounting a BTRFS filesystem thats made of 2 devices.

For example my data volume is made of md125 and md124

I can mount them like this usually

# mount /dev/m124 /mnt

or

# mount /dev/md125 /mnt

However if something happened you can do this before hand: # btrfs device scan

If that fails, you can try this:

# mount -t btrfs -o device=/dev/md125,device=/dev/md124,ro,recovery /dev/md125 /mnt