Hold down the Power button for several seconds or press the Restart button. I have been getting this error message on start up now repeatedly on two different computers (PowerBook Pro and 2.66 iMac) running OS 10.54. New OS has been running without problems for several months until now. Apple icon appears briefly on screen followed by a black rectangle with the above message. I can start in target mode and have run Disk Repair which comes up with a "apprear to be ok" message. Any help would be much appreciated. thanks.