The dual SD card models can be a little tricky, in that they remember which card slots were active when the camera was turned off. If you turned the camera off with only a card in Slot 2, then by default, the camera will use Slot 2 on startup regardless of whether you have
Use Slot 1 First as your setting.
With the camera turned off, please remove the card from Slot 2, put the firmware updated card in Slot 1, then turn the camera on. Slot 1 will now be recognized by the camera as the designated SD card to use.
Please also make sure that you have no previous firmware versions on the card, as this also results in a failure to download.
Let us know if this helps. Good luck.