This is probably too late to be of use, but this was the most relevant search result when I searched "Access is denied", so I'm answering now, in case others have this problem, too.
I got an "Access is denied" error when I tried to use arcpy.Mosaic_management (mosaic tool) with more than one input
path1 = "C:/path/to/file/file"
path2 = "C:/path/to/file2/file2"
paths = [path1, path2]
first_raster = "C:/path/to/first_raster/first_raster"
arcpy.Mosaic_management(paths, first_raster, "FIRST")
When I changed it to:
arcpy.Mosaic_management([path1], first_raster, "FIRST")
I no longer got the error. (in Python, you can create a loop over the path list, and can quickly mosaic all the rasters together.
I don't know why the mosaic tool cannot handle more than one input when I try to use it, but I also recall having this problem in the past.
NOTE: I'm running ArcGIS 10.3.1 on Windows 7, and am not running anything on a server.