Re: Origin won’t recognise Sims 4 is installed on other drive.

@Annarubanana  Too bad that didn't work.  And yes, you can try manually copying the pack folders from your external drive to the new Sims 4 install.  Try one at a time, then tell Origin to repair, and see what happens.  The pack folders that matter are the ones labeled EPxx, GPxx, and SPxx that sit directly inside the Sims 4 folder in Origin Games.

@puzzlezaddict  Alright I’ll give that a go after it’s installed the current one. Thanks for ur help!

This isn't going to help you now as it's been a year, but in case anyone else runs into the issue.  I have sims 4 downloaded through steam, but wanted to be able to open it directly from the launcher, but since it's been downloaded through steam, it is not able to find the games and wants to download again. (My game/EPs are all purchased through origin, but I'm having an issue with receiving the free content for cottage living so I want to see if I have to open it through origin launcher directly) 

Stop the download, click the cogwheel under 'install' and click locate game.  You want to choose the folder 'The Sims 4' not the individual files.  Inside that folder will be EP01+, GP01+, SP01+, etc, but you will only select the containing folder with the name 'The Sims 4' Once you've selected it, the system will take a little bit to check the file integrities and will check all expansion packs as well.