My A74 is on version 2.0 firmware.  I have tried to update to V4.0 but when I try the following message comes up

'There is no media inserted in Slot 1 (there is), there is no update file in the media (there is), or the file is damaged' I have downloaded the firmware twice and same problem. 

To confirm:

SD Card has been formatted in camera.

File is exactly the same file name

File is in the root directory of the card 

I have had no problems previously as I updated up to version 2.

Think I must be doing something obviously wrong but just cannot figure out what.

For info, file downloaded on a MacBook