Error during CreateOject

I copied a script and netCDF file from my C: drive, where it works, to an external NTFS drive (F:) where it fails. I created the same directory structure on F: so that the only change to the script is the drive letter (C to F). When I run the script on from F: I get the following error:

Traceback (most recent call last):
File "F:\mk_programs\esri\netcdf\dave_nc\code test file 2.py", line 41, in <module>
NetCDFFileProp = arcpy.CreateObject("NetCDFFileProperties", NetCDFFile)
File "C:\Program Files (x86)\ArcGIS\Desktop10.1\arcpy\arcpy\__init__.py", line 1217, in CreateObject
return gp.createObject(name, options)
File "C:\Program Files (x86)\ArcGIS\Desktop10.1\arcpy\arcpy\geoprocessing\_base.py", line 379, in createObject
self._gp.CreateObject(*gp_fixargs(args, True)))
RuntimeError: Object: CreateObject f:\mk_programs\esri\netcdf\dave_nc\ppt.nc not valid

I've compared the two scripts with software and the only differences shown are the drive letters. Any thoughts on what's going on? Does the fact that it's on an external drive effect anything?

Thanks,
Mike