Loading a game like Sims 3 from an external drive will almost always be slower than loading it from an internal drive. The connection to the external is unlikely to be as fast as the internal one, but perhaps more importantly, your Surface has a (very fast) solid state drive, whereas your external is likely a mechanical drive (HDD) and will run much more slowly.
This is not always a problem for the game files—you might be able to have the game itself installed on the external and only see small increases in loading times. But when your save itself is loading from an external, the effect can be much more pronounced, especially if the save is large in terms of absolute size.
Having said that, it's possible that this save is corrupted and can't load properly. Error 12 is an out of resources condition, usually but not always referring to memory, and saves running close to the game's ~3.7 GB RAM limit can become corrupted even without throwing an error, although they're usually fine. So while loading the save from the external drive complicates things, it may or may not be the underlying issue here.
If you want to test, try starting a new save with your current setup. If it doesn't work, then the issue is likely that your user folder is on the external, or perhaps something about the way you set things up. If it does work, and loading times are good enough for you, then you can leave the files where they are and troubleshoot the save itself.
By the way, I'd never store my game or my user files on an HDD, just because of the load times. And I'm not sure you need to keep your saves on the external anyway. Error 12 doesn't have anything to do with storage, so unless you wanted to offload data for other reasons, moving the user folder was unnecessary. If you do want to free up some storage on your internal drive, you can always transfer your backup saves and other data like screenshots.
