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.
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.
# rpm -qa | grep grub
What could possibly be wrong?