It might be interesting to determine if the problem is indeed related to running as the SYSTEM account. Maybe try following the instructions below to figure out if the issue is a user-based issue:
Thanks. The script did need to run as the current user, sorted now.
Just need to figure out the Relevance now to check for the existence of a particular file in a logged on user’s mapped drive. Running the relevance debugger on a target machine the relevance is False, but the machine still appears in Applicable Computers.