I found the following instructions online and they worked for me, so I thought I'd share. These instructions mean you don't need to turn off or disable or uninstall one drive, which is handy if like me you need it on your computer. I have a PC, so I don't know if these work for the MAC or not.
As always use these instructions at your own risk.===
Make a symbolic link.
1) Move the entire "The Sims 3" folder to where you want it. For example in D:\Sims3 - or, if you're like me, to "D:\The Sims 3". You need to make sure there is not a "The Sims 3" folder behind in the previous location, or it won't work. You DO, however need to leave the "Electronic Arts" folder behind.
2) Open a command console with admin rights (UAC)
a) Click the start button
b) Type CMD in the Start Search box
c) Press and Hold down [Ctrl]+[Shift]+[Enter]
3) Type
mklink /J "C:\Users\username\Documents\Electronic Arts\The Sims 3" "D:\The Sims 3"
Please note the inverted commas around the directory locations and replace username with your username. Obviously if you've saved it to a different letter drive "X" for example, you'd type in "X:\The Sims 3" instead of "D:\The Sims 3".
This will make a junction (symbolic hard link) to "D:\The Sim 3" from the original location. It recreates the folder you moved in the first step, but now as a symbolic link. It is like a folder level shortcut. Windows and all programs will think the files are still in "C:\Users\username\Documents\Electronic Arts\The Sims 3", but they are really in "D:\The Sims 3" now.
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Original Post I saw:
https://gaming.stackexchange.com/questions/34702/how-do-i-change-my-sims-3-installed-content-and-saved-games-locationAlso in:
https://forums.thesims.com/en_US/discussion/731492/moving-the-ea-document-to-a-different-driveIt works for me, game files are on my D:, my onedrive has space for my work files again and it also worked for Sims 4 ... replace Sims 3 references in your commands for Sims 4 and I'm happier than a tribble that's just found a secret stash of grain.