Hello. We were just trying to do the same thing today and had problems.
We had network attached storage mapped to a network drive ( Z: ) which required authentication.
The solution we found was as follows:
1) Ensure the tomcat (we're using tomcat) service runs authenticated with the required credentials to access the drive (For example Administrator etc).
2) (this was the bit we were stuck on for a while) in repository.properties z:/alf_data didn't work. We has to use the full unc path, //nas1/blah blah/alf_data
hope this helps some.