Server :: Create A "slow To Update" Raid - To Keep Spindle On Second Disk Spun Down?

I have a small SDD system disk and a large data disk on a server based on fedora core. It is organized this way so that the data disk can be spun down most of the time, reducing noise and heat.I'd like some way to periodically create a bootable backup of the system disk on the data disk, so that if the SDD goes belly-up, I lose minimal data and can very quickly bring the system up.Some thoughts:1. I know I can create a partition on the large disk and make the system boot from a raid0 mirror.However, the constant writes to the system disk (e.g. by the journal daemon) will ensure it never spins down.2. I can do a periodic format and cp -ax to the "copy of system" partition, but that doesn't leave the disk bootable because it needs changes to the /etc/fstab, and possibly a mkinitrd to become bootable.3. Or something like boot from a TFS (translucent filesystem) where base layer would be the raid, and the writable layer would be on the SDD, with periodic pushes of the writable part to the base layer (I'm not sure this is even possible).

4. In a virtual world this could be managed by appropriate location and use of snapshot images, but I'm not in a virtual world - as far as I know :0).Ideally I'd like something like raid that only brought the copy of system disk into sync with the running system infrequently (perhaps every few hours). Can anybody suggest how to do this?