how many times can you spot the letter "t" in:
q: grub2 w/ raid5 and btrfs "error: unknown filesystem" i have a virtual machine (in esxi 5.1) running ubuntu server 12.04. when i try to boot my system i grub2 tells me "error: unknown filesystem". i've tried booting a live cd then reinstalling grub from both a chrooted environment and by supplying the --root-directory option. i've even tried a boot-repair cd (results here) and reinstalling grub. whatever i use to fix it, i'm getting the error "path `/boot/grub' is not readable by grub on boot. installation is impossible. aborting."
to complicate things i'm running a raid5 array (5 disks) using mdadm and btrfs for my filesystem. the btrfs tools that ship with ubuntu 12.04 are really old. i've added ppa:cjwatson/grub (currently grub2 ver 2.00-2) and ppa:yofel/btrfs (currently btrfs-tools ver. 0.20~git20130524~650e656-0daily13) to my software sources to get more up to date utilities, with no help. the system was previously working before switching from a 4 disk raid0 setup. i think this may have been the first reboot since the upgrade, but it was running fine for weeks using raid5.

a: grub can't actually boot from anything but a raid 1 or raid 0 mdadm device.
you will need to add a /boot device somewhere and just set that up in raid 1/0/raw.
i would do something like this
the 2 new volumes below {a,b} can be file backed but should be on separate physical disks. 
/dev/sd{a,b}1 mdadm raid 1 /boot (2 new disks (500mb or so)) 
/dev/sd{c..g}1 mdadm  raid 5 / (current disks)


user
In the text, there are ninety-six "t"s.
assistant