Anderson, Steven D. and Rodger C. Young, “The Remembrance of Daniel’s Darius the Mede in Berossus and Harpocration,” Bibliotheca Sacra 173 (July-Sept. 2016): 315-23.
These authors argue that there is extrabiblical evidence for a king Darius prior to Darius I found in Berossus and Harpocration. This Darius fits in time period and position the Darius the Mede mentioned in Daniel. The most interesting evidence comes from Berossus, who wrote: “Cyrus at first treated him [Nabonidus] kindly, and, giving a residence to him in Carmania, sent him out of Babylonia. (But) Darius the king took away some of his province for himself.” This would place Cyrus and Darius as contemporaneous rulers. The authors say in a footnote they are inclined to identify this Darius with the Cyaxares II found in Xenophon’s Cyropaedia, though this is not argued for in this article.
I have been inclined to see Darius the Mede and Cyrus the Persian as two names for the same person, but I find the argument of this article intriguing.