An Active partition is the drive volume that an x86 computer boots from. It must contain the startup files and must be a primary partition on a basic disk. So you can't have an Active label on a portable drive.
Also an Active partition is not normally marked as such in Disk Management, unless you make a change in the configuration.
If you are trying to set up a removable drive as the bootable OS, you can't do that. If you are using the removable external drive for data storage and just have it active, it should be a Basic Disk with a simple volume, meaning the complete volume in contiguous on one physical HDD.