How to dual-boot two separate hard drives with OS already installed?

Solution 1:

Here is a simple way.

1. Insert both hard drives and find which hard drive the system boots into.
2. The OS which gets booted will be managing the bootloader for the system
3. Install EasyBCD (a free, easy to use software to manage your boot menu) You can download it from here: http://neosmart.net/Download/Register
4. Open EasyBCD and choose 'Add a new entry'
5. Choose the type of your operating system, specify the partition letter, and Save changes.

Restart your computer, and the next time you will see the bootmenu asking for which operating system to boot into.

Note: If your bootmenu time is set to zero, the boot menu might not show up, so increase that time, it can also be done in EasyBCD.

Solution 2:

So you have two hard drives, both have independent and bootable operating systems when connected individually?

The answer here is to use your system BIOS/UEFI utility, which exact details vary quite a bit based on the manufacturer and implementation, but generally one or both of these methods apply:

1) On demand selection - Most computer have a way of selecting the boot device on each startup, often called "Boot menu" or something similar, the exact key combination to bring up this menu varies but it is often ESC, F2, F9, F10 or similar. This should bring up a utility or boot menu allowing you to select a drive to boot from. This is a one-time setting and will revert to default on the next startup.

2) Changing the BIOS boot priority order - This is done inside the BIOS/UEFI settings of the computer, again this varies by manufacturer, but there should a boot order menu entry allowing you to select which devices to boot in what order. You would change this to the be the order you wish such as:

1) Optical Drive
2) USB Removable Drive
3) HDD 0
4) HDD 1
5) Network

and you would just change the order according to your need. It will search for a bootable device starting from the top and going down the list, you change this order as needed. This is persistent and survives a reboot.

Just a few references... there are literally hundred available on Google by searching "change boot order" or "select boot device" or similar terms. You could narrow it down further by adding your model of PC, such as "Asus m5a97 select boot device".

http://www.boot-disk.com/boot_priority.htm
http://www.howtogeek.com/129815/beginner-geek-how-to-change-the-boot-order-in-your-computers-bios/
https://www.lifewire.com/change-the-boot-order-in-bios-2624528