How to tell which Drive is active

[asian_tempation]

I have 2 3tb hdd in my PC( one active and one empty), and I want to put one in an external hdd enclosure as a backup, but I can't tell which one is the empty one and which is active. When I try to only connect one hdd via sata, it says "connect a boot device" for each drive. How can I tell which one is active?

 

[RealBeast]

Did you already try disconnecting one at a time and get the boot device error on each?

If so, then you probably installed Windows on one while the other was connected and will need to move the system reserved partition. Take a look in disk management to see where the SRP is located, and you may want to post a picture of your disk management here as it will be easier to answer your question.

An easy way to create or move an SRP is using EasyBCD.