Periodically, people will ask about the options for installing different versions of OS X on their systems. Though installing a newly released version of OS X is possible and recommended (at least by Apple) for all supported Macs, there may be problems if you decide to install a version of OS X that was released before your Mac was issued.
Many times OS X will come with machine-specific driver modifications that will not be present if an older OS is installed, and therefore even if the OS boots properly you may have problems getting the system or its components to work, or work stably. Apple has a couple of knowledgebase documents that cover the versions of OS X that were included with various Mac systems over the years, so be sure to check your system against these documents to ensure you are installing a supported version of OS X.
- Mac OS versions and builds included with Power PC-based Macs (since 1998)
- Mac OS versions and builds included with Intel-based Macs
There are a number of instances (some more viable than others) as to why people may want to try this, or at least inadvertently end up doing so, but despite this, you should not install an OS version that was released before your system was released. Here are some possibilities people may encounter:
Need to reinstall but only have older discs available
One common reason people try installing older versions of OS X is they need to perform a reinstallation of the OS but do not have the gray system restore discs that came with the computer, which may be especially true if the system was purchased second-hand. In these cases or even if people have a newly purchased machine and have just misplaced their restore discs, they may try installing a purchased retail copy of OS X that either is an entirely different release (i.e., Leopard vs. Snow Leopard), or that is a prior version of the same release (i.e., OS X 10.6.0 vs. OS X 10.6.5).
In many of these instances the OS X installer will likely prevent you from installing the OS; however, there may be times when it will progress. Unfortunately Apple is relatively reluctant to hand over extra system restore discs, but one option may be to purchase a new retail copy of the latest version of OS X (the retail box should have in print what version of OS X is on the install disc), and provided your computer meets the basic system requirements then it should properly load the new software.
Have software that requires an older OS version
Many times developers will require a specific version of OS X in order to run their software, so people may be tempted to obtain an older version of OS X for their new Mac and install it to run the older software.
If this is the case, your best solution is to wait for the developers to release an update to their software. Granted there are times when intermediate updates to an OS version will break some programs, but only downgrade to the prior intermediate release if that version supports your system.
Migrating a clone of an old system to a new system
Not only can cloning be used to back up a system, it can also be used to migrate to a new system without going through a full setup process. When doing this, however, some people may boot their new systems to Target Disk mode (immediately hold the "T" key after powering up) and clone their old system to it, inadvertently copying an older and unsupported OS version.
When migrating an older system using this method, be sure to first fully upgrade the OS to a version that supports the new computer's hardware. If this cannot be done, then use Apple's Migration Assistant to transfer user accounts and data to the new system.
Booting off an older computer's boot disk or clone for diagnostics
When performing diagnostics on your boot drive you might need to boot off an alternative boot source such as the OS X installation DVD. One alternative to the installation DVD is to use another computer's boot drive with that system in target disk mode; however, doing this may result in the system not booting properly and could even harm the other computer's OS installation.
If you ever need to run diagnostics on your boot drive, instead of booting your system off another computer's hard drive, start up your computer into Target Disk mode and then diagnose and repair the drive from another system as an external FireWire disk.
Beyond specific needs for installing or otherwise booting off an earlier OS version than what came with the system, people may just be curious as to what will happen. Though Apple does not support running an earlier version of OS X than what was shipped with the system, there is a possibility that potential incompatibilities will check out and the system will run the older OS software. In these cases, enjoy tinkering but be aware that any problem experienced could be the result of an incompatibility.