The RAID 5 is a NAS, so it is also external.
I am running off of a usb drive for many reasons.. I like to keep my internet ‘adventures’ away from my main machine for many reasons:
1. Security (I can always turn off the network connection for the VM only and cleanse the OS),
2. Snapshots (I can always revert the OS to a previous instance – so if something gets screwed up, it takes a minute to get back to normal),
3. Portability (I can take it with me when I am on the road instead of taking 2 laptops)
4. Speed (An external drive runs faster than the internal drive because it is not worrying about other I/O operations of the main OS)
5. Privacy (I don’t have internet stuff on my work laptop)
It’s a slick system (except if you don’t back up) 
I wouldn’t change for anything.
Andy