How to prevent the system from accessing the harddrive?

Question

Hard disks

I want to make two separate systems on different hards, how to make sure that they do not have access to each other?

Answer 1

You insert one disk - one system is loaded.
You interpose instead of it the second disk - the second system is loaded.
Complete isolation.

Answer 2

If the systems are different, they have different OS - and they just won't mount unknown partitions.
If the systems are the same - prohibit mounting disks in the OS itself.

Answer 3

Encrypt partitions and / or power switch for hards.