I write a lot of VBA and VBScript code to start AUtomated Monarch sessions. When I first started years ago -a lot of the users I encountered had restrictions on their harddrive imposed by security. i.e. they could only write to two predefined directories. Try writing to the user's c: emp directory - this should not be locked.
Also - it may not be a writing to harddrive error - could be a set up or coding error. Be sure you specify a monarch log to write to; and check the log after you run the auto monarch session (it will give you any errors that Monarch encountered for the session).