Boot from sandbox or just use it as a backup.

[dnanian]

High quality external drives aren't any more susceptible to "usage problems" than internal drives, Fred.

If you end up installing an update that was incompatible with your system, you would indeed just boot back to the original drive. That'll effectively uninstall the update (because it was never installed on the original drive), so you can get right back to work... and know to avoid the update until whatever incompatibility was there had a workaround.

Again: a Sandbox is NOT a backup. Your user files are not present on it. They're on the original hard disk. You must have a real backup as well...