There are two main problems I see in identifying Darius the Mede:
1) the destruction of Persian records by Alexander the Great, so that all we
have left are the Bible and a few scattered records from other sources,
2) I suspect Cyrus the Great as having tried to erase records of Darius the
Mede to justify his rule over his erstwhile ally, or more accurately he may
have been a top general under Darius and now he wanted to justify his taking
over Darius’ kingdom.