Is this a problem starting???

[#1]

Booted up my main system this afternoon, a full desktop system, with a PATA Windows C: Drive,the boot drive, and a SATA 2nd drive. Was working fine yesterday, but today, after the BIOS splash screen, got the message Insert System disk. No floppys in the drive, and after a bit of investigation, noted the SATA HDD had promoted itself to the Boot Drive, rather than the PATA, that was set as the boot drive previously. Cannot think of what caused this to happen, and the system is normal again now, after I altered this back to as it was, with the PATA drive as the boot HDD. Anyone able to explain what caused this.

[#2]

Well last night tried booting, and same problem. Went into the BIOS, and the system had completely lost the boot PATA drive. Although the innards of this system have not been touched for ages, I pulled all the IDE and sata cables, and reseated them, as well as their power supplies. This seemed to cure the lost drive, and things appear to be OK now. Ran the seatools diagnostic, from a boot CD I have, and drive passes, although the file system comes up with critical errors for some reason, I put this down to turning off the PC a few times with this issue. Windows disk management tool seems to think the System partition is healthy.

[#3]

You might also want to try replacing the motherboard battery. Normally when the machine is powered off it still powers the clock and bios from the mains supply however in the event of a power cut the battery does the job (which is why they can last for years - longer than if they were in a watch). That does however vary from one board to another. Some will use the battery even when a power supply is active.
If you have a power cut in the night then the bios is going to be a bit forgetful if that battery is low!
I've only ever had to replace a motherboard battery once... thats how long they normally last but it certainly did make a difference after if only to the clock which is normally the first sign of a low battery.