[gentoo-user] Duplicate identical Hard Disk

[Joseph]

I have two identical HD in the box and want to duplicate sda to sdb;
sdb is not even partitioned. 

I think I could do:
dd if=/dev/sda of=/dev/sdb
but I need to boot from CD isn't it?

I would like the second disk to function as an identical bootable backup (just in case something will malfunction after emerge :-/)
So I would normally boot from sda but have an ability to boot from sdb, just in case)
So after duplicating the disk I would need to modify the grub, fstab on sdb hard disk.

I'm not even sure if it will do what I need.  What are my other options RAID-1?
RAID-1 will not protect me if something happens after emerge. 

-- 
Joseph