As per the title, I am repeatedly being asked to install a Firmware Update for Thunderbolt on my 27" iMac. I have no Thunderbolt peripherals plugged in. When I try to install, the iMac restarts, I get a grey screen with a progress bar, but no sounds. There are no messages to say that installation was unsuccessful, but the update reappears in the App Store after a minute or so. Any ideas would be helpful, as it's very frustrating.