Mac owners can use two versions of the OS X operating system at the same time, computer magazine Macwelt XL says. This would be done, for example, if certain hardware such as an external hard drive is not compatible with the newest version of the operating system, Mountain Lion. Mountain Lion requires a 64-bit core and is not compatible with 32-bit software.
To use Mountain Lion and its predecessors Snow Leopard or Lion at the same time, the old system must be copied to an external hard drive. Then the user can install Mountain Lion. If users hold down the Alt key while the computer is booting up, they can choose which version they would like to use. Those who do not absolutely need Mountain Lion can forego the update altogether. Apple has announced that it will provide updates for Mountain Lion's predecessor Lion for about two years to keep the system secure and up-to-date.