How to scan another HDD / Windows for product keys?

First of all let's bring some clarification into product keys scanning. 90% of product keys are located in Windows registry and not in files on your HDD. Even though, those rest 10% which are not located in the Windows registry - most likely will be located in a folder where software is installed (in "Program Files" folder in most cases) or in %AppData% folder. Recover Keys will automatically determine where those apps are installed and where to look for product keys. It doesn't really scans all drives like C:\, D:\, E:\, etc. but checks in the registry where exactly on your hard-drive particular Apps are installed.

It's becoming more interesting when more than one Windows operating system is installed on 1 PC. Or if you have connected external hard-drive with another Windows OS installed (bootable or non-bootable). To scan external Windows OS (not one that is currently running) you have to select "Scan another HDD / Windows OS" from "Select action" dialog which appears on program's start, demonstrated on the picture below:

Select action dialog of Recover Keys

And then select folder where external (not currently active) Windows OS is installed. Recover Keys will scan external Windows registry and files for product keys.