In SharePoint, you can use the business connectivity services to get data from various external sources in your lists and libraries.
Symptoms
When migrating lists or libraries with external data, the external data is not preserved.
Details
ShareGate Migrate does not create external content types, so the connection to the external data is not preserved during the migration.
Solution
If you create the same external content type at the destination, ShareGate Migrate will preserve the external data in your lists and libraries when you migrate your content.