Owners of the current model of iMac — the “mid-2010” models featuring 21.5- and 27-inch screens — will be getting a firmware update via Software Update to fix what Apple calls “a hard drive issue that may prevent some iMac (21.5-inch and 27-inch, Mid 2010) systems from booting properly.” The issue is exclusive to the mid-2010 models, and the update requires the latest version of OS X, 10.6.7. The update is 767KB in size and can be downloaded directly from here, as well.
