Hey MisterFox UK,
Serato DJ will activate automatically when you connect a Pro controller or a supported ITCH controller. That is why it works fine whilst your NS7 is connected. Serato DJ Intro controllers like the Mixtrack Pro II can be used with Serato DJ but you need to buy a separate license.
serato.com
The reason your NS7 license doesn't necessarily carry over to you Mixtrack is to do with a licensing agreement put in place during the development of controllers. Numark and Serato decide that some controllers will come with Serato DJ activation whilst for others it is a paid upgrade. This makes it more accessible for beginner DJ's, they can buy an Intro controller that comes with a free but limited version of the software instead of having to pay more for a controller bundled with a full version of the software that they may not get the most out of.
If you do decide to buy a license for the software it will remain activated on your computer when any supported controller is connected.
I hope this answers your question,
Cheers,
