I have a Dell XPS 15z laptop, spec. is i7-2640 processor, 8GB memory, 750GB hard drive. The problem is that it often freezes and when restarted it suggests no hard drive, "No boot device found". If I go into the BIOS there is no hard drive, if I boot into the diagnostics screen (F12) and run thr tests on the hard drive it comes up as passed, if I then restart the laptop it will boot into windows 7 Ultimate. Evertime it freezes it won't boot after a restart unless I run diagnostics then it will start OK. My thoughts are that it is hard drive, although all tests I have carried out on it suggest its OK, one of the times it booted into windows I ran the Seagate seatools and it was fine. Anyone any suggestions as to the cause?