That was the problem it all started with. The new Mac Pro will not recognize the 10.5.1 CD from my 1 year old Mac Pro. Not even if the 10.6 was pulled out of the Pro. And BTW, the new bays are longer than the old (mine) ones. In total I had to upgrade (download) 1650 meg and lucky I could do it a lot at 1 MByte/sec. Paul Moortgat On 11 Feb 2010, at 05:15, Scott Buntin wrote: > > 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.