Externally Stored Containers
There is no need to move externally stored container data during a data migration. Container data is data and just like any other kind of data it stays on the production server during the migration process.
When you are migrating a file, you are moving a clone from development to production. A clone is created on Development and passed to the Production server. The clone has no data at all. The migration process on the Production server takes care or importing all data from the production files. The links to the externally stored container images are just another kind of data and therefor is imported into the clone or clones just like any other kind of data.