iMac: Downgrading Mac OS ROM Can Cause Start Up Failure

All iMac models have a file in the system folder called Mac OS ROM that contains information necessary for the proper start up of the computer. The version of the Mac OS ROM is matched to the individual iMac model and caution should be taken to avoid installing an earlier version of this file.

The iMac Update 1.0, which has been widely distributed on many third party software CDs for use on early iMac models, can be accidentally installed onto more recent iMac computers overwriting the more current Mac OS ROM file. This can cause the iMac to behave unpredictably or start up incorrectly, sometimes indicated by a blinking folder icon.

If you do install the iMac Update 1.0, you must replace the Mac OS ROM file with the version included with your computer, or a later version. To replace it with the proper version, do the following:

1. Start up from the Software Install CD that came with your computer (insert the CD and start up while holding down the C key).

2. Double-click the System Folder in the Software Install CD window.

3. Drag the Mac OS ROM file from the System Folder on the Software Install CD to the System Folder on your hard drive and click OK.

4. Restart your computer.

Note: The installer included with iMac updates later than version 1.0 perform a system check to prevent them from accidentally downgrading the Mac OS ROM file.

The Knowledge Base article 58174: "iMac: When to Install Available Updaters" has a complete list of the available updates and which ones should be installed on each iMac model.