pixiexhicks Do you want to move all of Documents to the other drive? If the other drive is external, this is not a good idea, but you can move it to another internal drive if you'd like. The error you're seeing is likely OneDrive-related, so you'll need to stop syncing or get rid of OneDrive altogether first:
https://answers.ea.com/t5/PC/CURRENT-ISSUE-Sims-4-saves-user-content-disappearing-PC/td-p/12475370#Remove
Once the location of the Documents directory out of OneDrive, you can further move it elsewhere.
The alternative is to create a symbolic link for the Sims 4 folder, or the Electronic Arts folder it's in. This works equally well on internal and external drives. Here's how:
https://crinrict.com/blog/2020/02/moving-windows-documents-folder-to-external-drive-via-symbolic-link.html
The catch here is that if the standard command doesn't work, write out the full path to the folder (in its current location, before you move it) rather than %UserProfile\Documents% as is in the guide. You're looking for the Location under Properties > General.
If you need more specific help, let me know which approach you're using. For the symlink, list the current location of the folder you want to move and the place you want it on your secondary drive.