That’s a fantastic idea. I’ve had the same thought myself.
One challenge with making it portable is that you need something that will work on any machine you plug it into. If there’s an emergency and you need the data on there when you don’t have your main computer, it’s likely that the machine you plug it into isn’t running Plasma. For this reason I think a hardware-encrypted flash drive with physical number buttons on it suits the use case better. That way you decrypt it with your fingers, and then the contents are readable on any random Windows, Mac, Linux, Android, iOS etc device you have to plug it into.