So... you want to move the users' home folders to the external drive? Or do you want to only allow the users read/write access to the external drive, and read-only access to the internal drive?
Users are not limited to placing "data" in their home folder -- they can put it anywhere. Each and every application has its own "default" place to put data, but the user doesn't have to follow that application's recommendation. Some like to put it in the home folder, some in a certain data directory specified by the application which may or may not be in the user's home folder, and yet others like the Desktop as a "data repository."
If you could elaborate a little more on what exactly you're trying to achieve, maybe we could be of more assistance.