Open Origin and go to Preferences (click on Origin, top left next to the Apple logo). Make sure you are under Installs & Saves.
Enable Keep legacy game installers and change the location of the saved installers (disk images) to somewhere easier to find*. I suggest creating a new folder in your Documents folder and calling it “Sims 3 game installers”, or something similar.
If you haven’t ticked the box but downloaded the game you can do the following. This has to be done BEFORE you install the digital download as the disk image and mounted installer disappear as soon as you’ve downloaded the game:
- download the game from Origin
- when the installer window pops up ignore it, open a Finder window and navigate to the following:
Macintosh HD > Library > Application Support > Origin > DownloadCache
- in that folder will be the disk image for your download. The file name will start sims3_ep9 (with the 9 indicating the EP, this number will change depending on the EP you have downloaded). Select, but don’t double click, the file then click on File > Duplicate.
- wait for the file to be duplicated then move it to another location on your Mac.
- you can now carry on with the install, safe in the knowledge that you have copied the disk image for future use.
- when you want to reinstall the EP simply double click the moved disk image and it will mount the installer on your desktop, ready to install.
By saving your Origin disk images you won’t have to waste time re-downloading the digital EPs and SPs in the event of a reinstall.
If you are installing on a new Mac you don’t even need Origin installed for this to work. Just copy the disk images onto an external drive, plug it into your new Mac and start installing.
*Thanks to godzillasims for pointing out this option to me 😳