My two bits,
Using a flash drive (thumb drive) as the boot has limited feasible usage. I have built this way as a security method. However, thumb drives, as I recall, are prone to failure more so than regular hard drives. As to what gets moved, the grub can be the only thing so long as it knows where to find the boot partition for the image. There is more technical info ofc but that is the basic jist. Given that raid 1 is mirroring the four partitions on each drive it is, by it's nature, the protection against failing hard drives as when one drive fails the other is happily booting thus giving you the opportunity to change out the failed drive. Another option is to keep a recovery thumb drive with the necessary Raid tools for your system. That is the beauty of Linux - options. I would not recommend regular booting to thumb drive. HIH.
-- Fred