Not everyone uses a single OS, some collect them like trading cards and switch between them constantly.  Usually this entails a certain install order and the use of something like LILO or a changed config file.  ExtremeTech has an interesting and simply little project that will allow you to use a physical switch to choose the SATA HDD you boot off of.  It is not an on the fly solution, the PC does need to be turned off to switch between the drives, but it can certainly simplify the boot process.  It does require a bit of soldering, so make sure you are comfortable with all of the steps before you start.

“If you have a motherboard with at least two SATA hard drive connectors and have at least two SATA drives, you can build a low-cost physical switch to select multiple operating systems.

Once you build and install the switch, all you have to do is select a switch position before starting your PC and your computer will automatically boot to the selected drive. It works by switching only the power between the drives. In order for this to work, you need to enable booting to your SATA drives in the BIOS (CMOS Setup).”

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