trying to put sims 4 on external hard drive

okay so i've basically ran out of storage because i only have a 256gb mac, so i got an external hard drive to put my stuff on, mostly my sims 4. i backuped my mods, tray, and saves, and then i uninstalled origin and ts4 because i had to take it out of my documents folder. i went to reinstall ts4 after uninstalling and reinstalling origin, and when i tried to change the location of the download, my hard drive wasn't an option. i know space isn't the problem considering it's 2tb and i just got it, so there's not much on it other than my mods. it's a laCie hard drive if that helps??