Could be a number of causes for this.
This can happen if you are on a Mac and:
a) Have it configured to use another drive other than the system drive for the users folder
b) have the system drive named with a special character, for example
it can also happen if you change the connection method to the internet from a hard wired connection to a wired connection for example
or if you have some sort of VPN Software that creates a virtual network interface that changes the Physical Address