SageSnel Try installing the game to your internal drive, moving the files to the location you want to use on the external, and "downloading" it again. The EA App may simply verify the files already there and finish the install process.
Your external drive needs to be formatted as NTFS. To check the format, and change it if necessary, click Windows key-X and choose Disk Utility. Formatting the drive will erase its contents, so make sure to move anything you want to keep to the internal drive temporarily.
As a side note, you should never install a game into Documents. Almost any other location is fine, including the default C:\Program Files\EA Games.