Hard drive conflicts?

All of this was mitigated by the ease with which extensions and the operating system itself could be swapped in and out: Instead of modifying configuration files as on other operating systems, extensions simply had to reside in a particular folder to be picked up. In addition, the Mac was perfectly happy to have two (or more) system folders present on a hard drive. Only the "blessed" system folder would be loaded at startup. So, when a new version of the operating system was to be installed, or a new application, the user could easily duplicate the system folder, perform the install, and then fall back if a problem resulted.