Started 12th August 2021
I turned on my PC (on Sunday 7th August 2021) and when I eventually looked at it, the BIOS was asking me to press F1 to enter. I wondered why? Anyway I entered the BIOS, and left it discarding any changes. The PC then went through the usual Linux boot menu with the default option of entering Windows 10.
It then did a BSOD due to "Windows Boot Device is inaccessible". Which sounded bad.
The problem is the Linux menu proves the main disc drive is accessible, as does loading some of Windows. Allowing a re-boot from the BSOD invokes automatic repair, which did nothing, and one can loop around these two all day.
Fortunately there are other options, a command line, from which I invoked SFC to check system files and chkdsk. Well chkdsk did find disc faults, possibly quite a lot of them because it managed to exit with "unspecified error".
How come I can run chkdsk on an inaccessible drive. I can also run the Ubuntu version of Linux from the same disc drive.
The people on Google say, roll back any Windows updates. It won't let me do that. I don't remember any recent Windows updates.
They also (less often) mention SATA AHCI mode. I don't know what that is, but I've not changed it in the BIOS.
So it looks to me like the insides of Windows are scrambled. SFC is reporting there are no system file faults, and by now chkdsk says the disc is OK.
It is admitting defeat and will cost a lot of time, but I give up and use the Windows reset option. Off it goes, but it does not fix the problem, it is still reporting inaccessible boot device etc.
Three days on, and I decide to find AHCI in the BIOS, I change the SATA setting from AHCI to IDE. Windows 10 will now run. Happily the old Windows installation is still there. However Windows begins doing a series of upgrades.
What to make of this. Did the BIOS suffer some sort of random error, which left it changing just the SATA mode. Or did the Windows boot error invoke the BIOS.
The general situation appears to be that Windows loads part of the way using BIOS code for disc access and then starts using its own drivers for a different SATA mode, at which point it finds it can't read the disc.
It did it again, power on, goes to the BIOS screen. No messing about this time, straight to the SATA settings and they're showing AHCI, change back to IDE and Windows boots correctly. This is a DOH! moment, going to change the battery on the mobo.
Yes, the battery on the motherboard showed 0.8 V, compared to the correct 3.3 V. I replaced it and reconfigured the BIOS (again).
"inaccessible boot device" means change the battery.
Started 12th August 2021