By design RadFileExplorer works only with virtual paths to folders inside the application root.
In order to configure RadFileExplorer to work with shared drives or physical paths you need to implement custom content provider. Such provider is available in the following KB article:
Use RadFileExplorer with physical and shared folder's paths
More detailed information on how to implement custom content provider is available in the following help article:
Using custom FileBrowserContentProvider
Additionally, please note that the latest moment of the page life-cycle where the FileExplorer's path can be configured is the Page_Load
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