We are getting 'Cannot specify both an external ID reference Owner and a salesforce id, OwnerId' error while upserting account record. Any idea?
I believe this is as simple as the error message is giving you. You cannot specify an External ID field and the Salesforce ID in Updates or Upserts from Dataloader - because imagine if in your data the Salesforce ID referenced one record, but the External ID actually was on a different record; which one would it upsert? It would have no choice but to blow up.