There are numerous plugins that will let you copy files at an operating system level (e.g., Windows Explorer or Mac Finder). The biggest issue often is permissions. Remember that FileMaker Server runs as a service and the user that it runs under is often rather restricted for security purposes. Note that because of this generally FileMaker Server generally can only manage files in the \FileMaker Server\Data\ folder. This really is a security protection to keep FileMaker for overwriting files from other programs or operating system files, etc. You will have more success if the mounted drive uses the same user as FMS is running on. Also, getting file paths correct on network volumes can be a challenge as well as getting them mounted.
I have not even gotten as far as file permissions. I do not know how to use the script master copy function to try and use the file. How do I call the copy function within a script, they are only available in calculations? Do I create a variable and use a calculation with the function in it to set the variables value?
Plug ins are typically implemented like this;
Set Variable [$result ; value: ExternalFunctionCallFromPlugInHere ]
The act of assigning a value to $result, which many plug ins use to report success or failure type results that you can then test for, invokes code in the plug-in.