OP, you were on the right track moreso than any of the suggestions that followed.
The first 2 parts of the error code tell you the previous and new dash versions respectively (in hex). So, you were going from 12625 to 13599 when the update failed and it's therefore obvious why it failed.
Since you have a pre 0251c dash, you need to flash your drive with 7485 stock firmware and then apply the 13599 dash update from USB stick.
That's providing the 74850c is the original drive. It may not be!