This update includes fixes that improve performance and stability for Thunderbolt.

The iMac EFI Update will update the EFI firmware on your iMac computer. When your iMac restarts, a gray screen will appear with a status bar to indicate the progress of the update. It will take several minutes for the update to complete. Do not disturb or shut off the power on your iMac during this update.

After the update Boot ROM version = IM121.0047.B0A