You perform copy or migration of activated Windows OS, and after booting from target disk, it asks to perform activation once again.
This happens because Windows operating system has noticed recent change of hardware.
When you make a significant hardware change to your computer, such as upgrading the hard disk and memory at the same time, you might be required to activate Windows again. This is designed behaviour of the Microsoft Windows.
For more information, see the following Microsoft KB article: Activating Windows: frequently asked questions