When exporting a workflow that contains a File Transfer activity which is sending to a Secure File Transfer Protocol (SFTP) site via an External Account that upon import the File Transfer activity inherits this reference to the External Account even though the particular account in question doesn’t exist within the new environment.
Additionally, the transfer activity cannot be updated to select a new External Account.
Currently there isn’t a resolution, however CAMP-38001 has been opened with R&D to address this with a long-term fix.
Meanwhile, the workaround is to delete the imported File Transfer activity and recreate it. This will allow selection of the external accounts.