When trying to migrate CLM content with custom lookup fields on any of the related records, the migration fails with a Copying of media file failed message. This is followed by the field names and ids of the records that are failing.
If the lookup in the source org references a record that doesn't exist in the destination org it will fail. For example if you have a lookup on the key message object to the CLM presentation object the CLM presentation record being referenced would need to already exist in the destination org.
This configuration should be avoided where possible, if it is not possible to avoid it you need to make sure the that the records referenced in the lookups exist in the destination org before migrating.
CRM Help Documentation: Migrating Content
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