Is it that only the C drive appears in the File picker or can you also see the mapped drive letters, e.g.

If you can see the shared drive letters, is it that the sub-folders and files not being displayed? Depending on the network topology there can sometimes be a long lag before the sub-folders and files in the root directory of the share are displayed.
You may also want to try configuring the node with the UNC path for the file , e.g. instead of typing T:\myDir\foo.csv use the \\host-name\share-name\file_path format.