If you need or want to have multiple operating systems on a PC, you should be able to access them separately. Some operating systems will take over all of the hard drives, and you may lose data or have data corrupted. (This is the voice of experience speaking!) So I came up with a simple fix that involves an external multiposition switch. This is easier than disconnecting the signal ribbon cable and the corresponding power connectors.
Having two distinct operating systems on separate hard drives, I made a two-position double-pole switch operate so that it chooses between operating systems. The figure shows the connection. You can hardwire the drives or use connectors. You can switch as many hard drives as your system has mounted in the PC.
Two voltages are present on the power cable that's connected to each device (floppy drive, hard drive, CD, etc.)—+12 V and +5 V—as well as ground. The ground wires (usually two, colored black) connect to the frame of the PC and each powered device.
While keeping some of the powered devices connected normally (floppy and CD), the multiposition switch permits access to the hard drive or drives in a simple and convenient way. When changing hard drives (operating systems), the PC should be shut down to clear the RAM.