We experienced the same problem. As a work around what we do is created a new library called TransferLibrary. Using library Services, copy the part from the Master library into the TransferLibrary. Zip up the TransferLibrary and email it to the mirror site. At the mirror site, unzip the TransferLibrary and use Library Services to copy the part into the mirror site library. Include instructions to tell which partition you want the new part transferred in to.
It is not the way it should be done but it gets the part into the mirror site library.