Just some thoughts on this. I guess I'd probably set this up with a removable caddy and a second internal hard drive....maybe with two partitions.
The internal drive could be the general purpose/engineering system with RollBack installed to swap between the two. The second partition could contain the data which might need to be accessed by all three systems.
The caddy would have the CNC system on it which I'd be inclined not to install RB on at all. This is simply because, as you point out, anything which has the potential to delay a mission-critical real time operation could be bad news. No disrespect to RB but I doubt if it is designed with real-time operations in mind. Having said that, I'm not sure that Windows is either
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Anyway, with the above, it could be set up such that if the caddy was plugged in then it would boot from that and if it wasn't then it would boot to your other systems.
Feel free to disregard this
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Graham