You can't use copy & paste (clipboard) between different libraries from different workspaces. Instead, you need to (temporarily or permanently) add the library to the same workspace. In the menu go to File > Manage Libraries and then use the "Add library definition file ..." button to add the libs.def file of your "source" workspace. Now, all libraries are available from this workspace and you can copy cells between libraries. When you are done with copying, you can use File > Manage Libraries to remove the libs.def of the source workspace again.