While master hard drive was being defragged, Avast gave "Virus data base has been updated" message, and slave hard drive wasn't seen when Defrag completed. At first I thought Avast may have caused this, but I now think Avast wasn't the cause due to subsequent events. Next I restarted the system after a couple of hours with everything appearing normal, copied some files and folders from slave to master drive using Explorer, but then all icons on desktop disappeared as well as task bar including start menu, shut down, etc when Explorer was closed. The only way to shut down was manually with power switch. Upon restart the slave drive was again missing. A few BIOS settings were changed and system restarted a couple of times with slave drive still not showing. Then another restart try resulted in a blank screen with blinking cursor in upper left corner with no error messages. Boot to floppy and CD worked a few times after which CD boot failed, and now only boot to flolppy and CD via external USB DVD drive is possible. During boot to either shows no drives found during boot sequence and finally no valid CDROM device drivers found. All drives show normally in BIOS, but IDE drives aren't seen during boot to floppy or CD in external DVD drive. I tried changing IDE and power cables to see if that was a problem, but results are same after changing cables. The occurrence during Defrag now appears just coincidence and a symptom of much more serious issues
This progression of failures seems to me to point to failing and finally failed IDE connection on mobo or other mobo failure. Could these kinds of failures be caused by CPU or PSU issues, or is it more likely IDE controller failure? The mobo also has Onbaord SATA-IDE Controller settings in BIOS with options of RAID, SATA and Disabled and is currently set to RAID which I don't use. Would changing this BIOS setting to Disabled effectively change those mobo connections to IDE connections and allow trying connecting IDE drives to those connections?