iMac EFI Firmware 1.4 Update addresses issues where iMac computers with ATI Radeon HD 4850 graphics intermittently stop responding and addresses wake-from-sleep issues in Boot Camp.

Before you begin, find out which version of firmware you have, see EFI and SMC firmware updates for Intel-based Macs.

Some USB and FireWire devices may prevent firmware updates from installing correctly; disconnect non-essential devices and use only an Apple keyboard and mouse to apply the update.

Quit all open applications.

Download and install the firmware update:
1. Download the appropriate update for your computer. MacBook Pro EFI Firmware Update 1.7 or iMac EFI Firmware Update 1.4.

2. The downloader will place the disk image with an installer in your downloads folder or the location specified in Safari General Preferences. If you downloaded the disk image via Safari, Safari will notify you that an application is in the image. Click OK to continue.

3. If "Open safe files after downloading" is enabled in Safari preferences, the disk image file called MacBookProEFIUpdate or iMacEFIUpdate will mount by itself. If that preference is not enabled in Safari, double-click the disk image to mount it.

4. In the mounted disk image, double-click the MacBookProEFIUpdate.pkg or iMacEFIUpdate.pkg file to start the firmware update process. (The installer puts the iMac EFI Updater in the /Application/Utilities folder by default and opens it automatically.)

5. Read the onscreen instructions and click Shut Down to proceed with the update. The firmware image will be loaded onto your computer. Your computer will shut down, but the update will not be installed until step 6.

6. Turn on your computer but do not hold the power button down. The screen will turn black and remain in that state for up to 40 seconds. The computer will emit a long tone. A gray screen will appear and a status bar indicates the progress of the update. Important: Do not unplug, shutdown or restart your computer while the update is taking place.

Installation may take several minutes.

1.After your firmware has been updated, your computer will restart.
2. After you log in, an alert will appear, stating the firmware update has been successful. Click OK to dismiss this dialog.
3. If the update to your firmware was interrupted, your computer will attempt to reinstall the update by itself the next time it is powered on. Do not unplug your computer during this process. Your computer will restart by itself when the EFI Firmware installation is complete.
4. If your computer does not restart, contact support for more information.

Important: The hard disk in an Intel-based Mac must be formatted with the GUID partition scheme to apply a firmware update. Intel-based iMac computers are shipped this way from the factory. If you have reformatted your hard disk, you should check which scheme your hard disk is currently using before applying a firmware update. See Firmware updates for Intel-based Macs require a GUID partition scheme for more information. If you try to apply the update to a hard disk that utilizes any formatting scheme other than the GUID partition, the update will be unsuccessful.

[Source: Apple Downloads]