Hello Apple I have bought a 2012 27'' iMac start of this year. Since I've bought it I have troubles with the Thunderbolt ports. As soon as you update a new version of Mountain Lion I have to dismount the external hard drive and then do the update as of this week I got the new updates and the Thunderbolt Firmware Update 1.2 is not being installed although I've been following your resolution http://support.apple.com/kb/TS4579?viewlocale=en_US&locale=en_US but this does not work either.
I have also a Mac Book Pro and don't have these problems at all. I've reset as well the PRAM, let the iMac 10 minutes shot down, took if off the power plug and took out the memory. Nothing of it helps. I believe that the Thunderbolt Firmware Update 1.2 has errors. If there is no resolution to this I'd rather prefert to wait for Maverick.