Can’t install Sims 4 expansion pack onto external hard drive

I installed the Sims 4 onto my Seagate 1T external hard drive a few months ago because I simply don't have space on my mac for games. Since then, it's worked perfectly so I finally bought an expansion pack on Origin but now it won't let me install it onto my external hard drive. There is still more than enough space on the drive and I created a folder for it before I tried to install it but I keep getting a message saying "Games cannot be installed in this folder" (which can't be true). I have tried changing the game library location for installs and saves but the same error comes up.