Sometimes Computer Management won’t let you use the entire SD Card. (By not letting you extend your primary partition.)
To fix this you need to clean the card up. Warning: It will erase the contents of your card.
Run the following in a cmd prompt:
At this point, find your SD Card. (Easiest to do by identifying its size.)
select disk X
create partition primary
format fs=fat32 quick
(Where X is your SD Card drive.)