I know that you can install the game, content and expansions to any drive you want. If your concern is to maintain the installation in case of computer crash here is a suggestion.
Attach your external drive. Prefeably Esata, Firewire, or USB 3.0 since they all have better speed and make game play possible without extreme lag. Once the drive is installed install a fresh or virgin installation of Sims 3 to your computer. However, during installation do not let the install wizard install the content folders to your main HDD. Instead tell it to browse for another location and pick your installed drive. You'll need to set up folders ahead of time, I did this on my backup drive one for each Game, expansion and Content CD so that I could keep them separated. I created a master folder that I called "EA Games" Each of the expansions and content CDs I stored inside this folder and in a sub folder which matched the title of the installation disk.
The install wizard will install the majority of its data to whatever folder or sub folder you assign it to. Once that is done, go to "Users>"Your computer name">my documents. You will see a folder called "Electronic Arts. This folder contains most of the active game components. You will need to compy the whole folder to your external drive. This is your backup. It allows you to keep a total copy of Sims 3 without having to reinstall in the case of a system crash. I like to put it on the other drive inside a folder marked "Sims (combined with date written in this format 12-2-2012, example)
If your computer ever crashes and you need to replace sims, you can install the first game only then copy this boder back into the primary location on C drive. If your external's letter remains unchanged you should be able to play the game from where you left off.
This may not be exactly what you're looking for. But it is the best I can offer from personal experience. I hope it does help you to some extent.
traveller004
