When the iMac Firmware Update application runs, it copies a file to the Startup Items folder in the iMac computer's System Folder. If the iMac is NetBooted when the updater application is run, it is not copied to the iMac computer's local hard disk, but instead to the System Folder "shadow" image residing on the NetBoot Server. Upon restart, the iMac uses the unmodified System Folder on the NetBoot HD.img. The file that should be in the Startup Items folder is not there, and the iMac computer's Firmware is not updated.
In order to run any Firmware update on an iMac or other NetBootable computer, you must first start up the computer from the internal hard disk, run the update, and restart from the internal hard disk.
Note: The issue described in this article may also apply to other CPU-specific firmware updates.