Write Protection

Hi All,  I am trying to load a firmware update onto a Sandisk Extreme Pro 32g SDHC card.  I am using a Mac, and the disk won’t load the bin file from my Mac.  I “get info” on the card and it states “Read Only”.  The cards (I have several - all with this issue), will record photos from the camera just fine, and they are transfered by Apereture and Light Room just fine.  I can’t seem to add things to the disk when connected to the Mac.  And this is how Nikon needs to do firmware upgrades, by putting them onto the card and inserting in the camera slot.  Any ideas how to get these cards to “Read & Write”?