Have a read through this article which describes some best practices and problems you may run into
http://support.microsoft.com/kb/207671[
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Do not use drive letters mapped to network shares. Not only are there only 26 potential driver letters to select from, but if you try to use a drive letter that is mapped in a different security context, problems can occur. Instead, you must always use Universal Naming Convention (UNC) names to access resources. The format must look similar to the following: \\MyServer\filesharename\directoryname\filename
Mapped drives are user-profile based, so unless the security context you are running your IIS application as on the test server explicity has this drive mapping, it's not going to understand what Z:\ is.