Failover boot setup

I am considering a failover boot setup which can tolerate any single drive loss. The primary system partitions (/, /home, /usr etc.) shall reside on a flash drive. /var and /tmp partitions shall be mounted from a hard drive. Additionally hard drive contains shadow copies of all partitions from flash drive; these should be rather static and can be kept up to date with RSYNC (?) plus it has boot preset to start using these partitions. In case flash drive fails, system simply boots off the hard drive (should be set up in BIOS). In case of hard drive failure system should still be operable to allow hard drive replacement. My questions are: 1. Do I miss something essential in such configuration (e.g. security)? 2. If hard drive is lost, the system shall boot with no /var and /tmp mountable; will it start up at all? Of course this is a recovery operation mode which should minimally allow another drive to be set up and mounted properly, but it should at least boot and start all services including ssh. The distribution I am looking at is Debian, but this is not a final decision.