Can't Boot newly created RAID1

I have decided to add another disk in raid1 to my system since the one I have is quite old,

I have replicated GPT from /dev/sda (old drive) to /dev/sdb (new one) then i created the raid array with mdadm and then I configured Grub2 to boot from raid instead of the single drive

in the load script i'm loading raid and mdraid modules but however despite the fact that i added them as GRUB_PRELOAD_MODULE to /etc/default/grub file and issued the update-grub command at boot time grub is still telling me that the modules can't be loaded

I dint know what I'm doing wrong...