Geralmente, é necessário ter um / boot separado, a menos que você queira inicializar o sistema em um dos dois discos RAID1 e, em seguida, remontar como md após a execução do sistema ou configurar um initramfs apropriado.
De wiki do mdadm :
Since support for MD is found in the kernel, there is an issue with using it before the kernel is running. Specifically it will not be present if the boot loader is either (e)LiLo or GRUB legacy. It may not be present for GRUB 2. In order to circumvent this problem a /boot filesystem must be used either without md support, or else with RAID1. In the latter case the system will boot by treating the RAID1 device as a normal filesystem, and once the system is running it can be remounted as md and the second disk added to it. This will result in a catch-up, but /boot filesystems are usually small.
With more recent bootloaders it is possible to load the MD support as a kernel module through the initramfs mechanism, this approach allows you to let the /boot filesystem be inside any RAID system without the need of a complex manual configuration.
Embora não seja sua pergunta, pode ser útil consultar o Boot RAID para mais informações sobre o uso do initramfs para iniciar um sistema que inicializa a partir de volumes md.