Currently I have a 27" iMac (mid 2011) running OS 10.8.2 and up until now I have been using the two thunderbolt ports to power two displays. One is a small TV running at 720p using a Thunderbolt to HDMI adapter while the other is a Thunderbolt display. Both have been running well and have had no problems but I decided to move the TV out of the room so there is now just the Thunderbolt display connected to the iMac. For some reason, the display seems to be underscanning the image on the Thunderbolt display with about a 1" border which makes it nearly unusable sitting next to my iMac as the size of anything on screen changes slightly when moving between the iMac and Thunderbolt display. If I reconnect the TV to the open Thunderbolt port with the use of the Thunderbolt to HDMI adapter, the Thunderbolt display returns to utilizing the entire screen area. I have reset the NVRAM to no success and tried restarting the computer and unplugging the Thunderbolt display to no avail.