Is there an error in the example given for inference of friends on the following docs page?
I had to place an @ before the prefix in order to get the example working.
EDIT::
So a good look at the TURTLE w3.org page states both PREFIX and @prefix are fine. But with PREFIX it appears there should not be a '.' at the end of the PREFIX line, so removing that works fine to load the ttl file as both a graph and an EXTERNAL source.
Thanks for the report. The update will be made.