[X4U] 10.5 instead of 10.6

[Scott Buntin]

On Feb 10, 2010, at 12:24 PM, Paul Moortgat wrote:

> He didn't remember exactly where he heard it.  He assumed it was from Apple.
> The Apple site <http://support.apple.com/kb/HT3964> doesn't mention anything about a lower OS.
> He told me that part of the OS is stored there and will block a lower OS.
> 
> Paul Moortgat


Your friend is wrong. The SMC (and  in older machines, the PMU or SMU) really doesn't care what you install. 

You can install any version of the OS that is greater than the version that shipped with the machine. You can install the version that shipped with the machine, but usually only with the DVD that came in the box (not the retail OS DVD).

If your machine shipped with 10.5.5, you can install 10.5.5 or greater. You cannot install 10.5.0 (the retail 10.5, for instance) and usually cannot install from a DVD that shipped with a different model.

When you run the installer for the "older" version of OS X, it will not allow installing unless you change the install options. You must either erase the drive, or do an "Archive Install". 

If you are trying to install 10.5 over 10.6, you should plan on erasing the drive, or be *very* selective as to what data to keep - Mail libraries, as well as some pref files created or modified in 10.6 may not work well, if at all, in 10.5.