Apple's has warned that the installation of iMac Update 1.0 has the side effect of placing a downgraded version of the Mac OS ROM file on the machine. This is likely to cause the iMac to behave unpredictably, or start up incorrectly.
iMac Update 1.0 is only intended for use with iMacs with 233 and 266MHz processors. This upgrade is not required, and is not compatible with 333MHz or slot-loading iMacs.
If you do install iMac Update 1.0, you must replace the Mac OS ROM file with the version included with your computer. 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.