test-casconnectivityuser.ps1 errors out with "The storage group you specified .... is a recovery storage group"

Okay, so this isnt going to work, however the error could perhaps be a bit more verbose as to why and how to fix this.

The script runs a get-mailboxdatabase, it then tries to add the new user to the last one of these returned, if you have an RSG this will be returned last, and thus fail.

In my case i removed the (unused) RSG then reran the script, and off we go, it worked!