... a dynamic disk is "tied" to the machine it's created on. When you connect it to another system, it's considered "foreign." But you can "import" it, and it will then be associated with the new system.
It's unfortunate that Microsoft offers the "convert to basic" option without an explanation => I've seen several instances where someone selected this option and lost all of their data. Once a disk is dynamic, there are no Microsoft tools that allow converting this back to a basic disk without loss of data (there are some 3rd party tools to do this).
But in your case, it's simple ==> Just select "Import Foreign Disk" and you'll be fine.