I have looked up previous posts about "Primary IDE Master not detected" but my problem looks a bit different. Since a few days my pc (desktop ASUS P5P800S, Pentium 4 w. 3.0 Ghz) does not always boot after switching it on. Sometimes it boots after several attempts, but sometimes I have to go to the BIOS screen and when I leave it, it boots. Note, I don't change any setting, just enter the screen, leave it and Windows starts. After this the pc runs without any problem. After standby it starts also correctly. In the boot sequence the secondary master and slave are detected, but then comes a "Third master Hard disk error" (which is the C:" drive and "No IDE Master H.D.D. detected!". The strange thing is that BIOS does detect the C:-drive: BIOS shows that the primary IDE master and slave are not detected. Secondary IDE master is the DVD, slave is the HD "E:" (Maxtor 160 GB) Third IDE Master is the boot drive "C:", a Maxtor 120 GB. I don't understand 1) why the C:-drive is detected by BIOS but not in the Boot-sequence and 2) why the system runs anyway after passing through the BIOS screen. The is not understandable reason for the change, I did not open the PC or modified anything It drives me mad.