origin says external hard drive not mounted

Playing the game off of an external hard drive is a tricky thing. It can cause issues with the game and the saves. The game and Origin normally want to look for your game on your computer's hard drive. You will have to go into your Origin preferences and change to where you want the games to install otherwise it will always install to your normal hard drive. (At least this is what is recommended for the Windows OS).

Personally, I would simply install the game on your normal hard drive and if absolutely necessary then move everything to the external hard drive and make sure that you change all game preferences to recognize the external hard drive. Once you get the game installed and before transferring, start it up and allow the normal Electronic Arts folders to generate to see if you have any other issues. I would not install Mods or CC until you make sure the game is functioning properly and Origin is directed to look for the game on the external hard drive.