MyDrive Connect should only 'operate' with the unit currently plugged in, regardless of whatever other devices it has known in the past. Are you referring to the login name in the upper right? If so, hit "Log out" and log in again with the account you have set up for the 520 (vs. the 52). When you start to type an email address, MyDrive connect may well show previous login possibilities depending upon your browser's 'auto fill' capability. That would have to be addressed in your browser.
Truth to tell, with your 520, you don't necessarily need to be using MyDrive Connect to update your device. In fact, when you plug in your 520, MyDrive Connect should advise you regarding WiFi use.
The 520 can be updated in two ways. First, you can connect it to a home or other WiFi source and download any updates that way.
The second method: IF MyDrive Connect is running in your PC and you connect your device, and although you will not use the MyDrive user interface at all, MyDrive completes the hardwired connection between your device and the PC as a TCP/IP connection to the servers at TomTom.