I have a Mac Pro with 3 2TB internal media drives, and a 4 (2TB) G-Speed Q external array which is formatted via RAID-5 to also be 6TB in size. What I'd like to do, is use the G-Speed as my primary media drive by connecting it with a 4x eSATA card, and then I'd like to setup the three internal drives to be striped as a software RAID-0 to be an equal 6TB drive to be used as my backup for cloning the external to. If I do this, and only use the software RAID when I'm cloning, can I avoid the CPU overhead that comes with a software RAID setup, or will I take the hit just to have the software RAID mounted?
The CPU overhead occurs when actually accessing the array, not for mounting it.
Do remember that with a RAID0 (simple stripe set), if just 1 of the disks goes bad, everything is lost. Also, please don't consider different volumes/drives on the same machine as a good backup strategy. Things like bad/failing RAM or a bad/failing power supply could corrupt any/all disks in the system. It may see like a remote case, but things like this do happen -- My Murphy loves to make fun for you. Depending on your specific needs, you may be better putting those disks in a different machine and making the copies/backups over the network. This scenario removes many more failure points.