Warning: I’m not a professional electrician. My plans and explanations could be wrong and if you follow them you might break your computer or get you electrocuted. The following project documentation is provided without any warranties of any kind.
Some people want to use multiple operating systems and often they do not want to buy multiple computers for that purpose. So they install multiple OSes on one computer. This works well if all the involved OSes are aware of each other and try not to interfere with each other.
In practice this is not always the case. There are OSes which always want to reformat hard drives of (to them) unknown file systems. There are updates, which rewrite the boot loader code in such a way, that the other OS is not bootable anymore. So I wanted a hardware way to disallow any interference between the two OSes.
The idea is, that there are two hard drives in the computer, with one operating system on each. Both are connected to the motherboard via SATA data connector cables, but only one at a time is connected to the power supply unit. If a hard drive does not have power, it can neither read nor write data. It can’t event power up its controller to tell the motherboard that it is there.
Note that we only need to switch the actual power cables and not the black ground cables. Ground alone does not provide any power and plugging in only ground is not harmful to the hard disk drives.
The parts I used are a SATA power Y cable and a 3xOn/On switch. On the tool side you need scissors to cut the cables and soldering stuff to put them back together in the way you want. Furthermore you need a continuity tester (present in most multimeters) to make sure, you soldered this correctly. If the wrong voltage arrives at the wrong pins of a hard drive, it will probably break. If you solder a power cable to ground, your PSU might break or even explode. Potentially other parts of your computer are also in danger. So please be very careful if you try this at home.