I am struggling with a multiple OS install of grub2. Currently I have just
CentOS 7 (/boot sda2) and CentOS 6.6 (/boot sda9) installed.

system is structured BIOS/GPT.

grub2 loads, menu comes up, but I cannot boot into 6.6, only into 7. The
error message is "unknown filesystem".

If I drop into grub2 command-prompt, "ls" lists all of the partitions
without a problem. If I try to "ls (hd0,2)" it shows the partition info
(happens to be xfs), and "ls (hd0,2)/" shows files. If I try to "ls
(hd0,9)", it shows "unknown filesystem".

Once booted into CentOS 7, I am able to cleanly see and mount sda9. I tried
rebuilding the sda9 /boot partition as ext2, ext4 and xfs, then
grub2-mkconfig and grub2-install, all with the same result.

I posted details on a stackoverflow question here
http://unix.stackexchange.com/questions/245655/grub2-cannot-see-filesystem-on-alternate-boot-partition

The BIOS boot and GPT must be fine, or grub2 wouldn't load. The filesystem
drivers must be fine, or it wouldn't be able to load (hd0,2). The sda9
should be fine, or it wouldn't mount once in CentOS.

grub install:

# rpm -qa | grep grub
grub2-2.02-0.17.0.1.el7.centos.4.x86_64
grubby-8.28-11.el7.x86_64
grub2-tools-2.02-0.17.0.1.el7.centos.4.x86_64

What could possibly be wrong?

-- 
Avi Deitcher
Follow me http://twitter.com/avideitcher
Read me http://blog.atomicinc.com
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