Why can't the custom object owner field be updated using Data loader when field type of "External id" is referenced?
|Knowledge Article Number||000199196|
|Description||Please note that DataLoader does not support upsert by using External ID for polymorphic fields, that is fields that can accept values from multiple table (i.e. User and Queue). All custom object (as Project) uses "Lookup (User, Queue)" for Owner (as well as Leads, Cases), so that is why it doesn't show up on. There are only a handful of Standard Objects that uses the datatype "Lookup(User)", such as Account (that's why it appears).