Aha! It seems like sometimes, for some reason, reinstalling the kernel packages renders grub-probe unable to detect the type of the filesystem for /, hence why it doesn't writ the btrfs-related bootflag...
-- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/752506 Title: new -8 kernel makes update-grub ignore writing subvol option for btrfs -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
