It's usually done with resource files. If it is already multi-lingual then can't you simply extend the method you have now for other languages? Converting a site to use resource files if it doesn't currently use them is a substantial effort.
Having the files on a different drive shouldn't make any difference.
Have you checked that the physicalPath variable contains the correct path?
If it still doesn't work, then it must be an NTFS permissions issue. Make sure that IIS_IUSRS has at least "read & execute" access to the folders on the E: drive, and reset the permissions on all child objects.
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