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I have had my desktop PC for a year and a half and no major problems have occurred, other than 2 minor BSODs in total that appeared only once and never again. Today, however, was a different day.

When I came home, I turned on the power strip and my desktop turned on by itself, something that it's not intended to do. It didn't show anything on the screen and HDD activity stopped, so I held down the power button and it turned off. When I pressed the power button again and waited for it to turn on normally, it did the exact same thing again. I then proceeded to force shut down and turn the PC on normally for a second time. This time, it turned on successfully, but a BIOS message popped up saying that time and date were not set correctly. So, I pressed F1 to enter setup but the system rebooted on its own. It then booted into Windows, but drive activity halted just before the spinning circle appeared. And you guessed it, 20 seconds later, a INACCESSIBLE_BOOT_DEVICE blue screen showed up.

Before looking anything up online, I ran chkdsk just to be sure that my HDD is not failing (I am planning to upgrade to a SSD, so I had not backed up anything). Chkdsk did not find any bad sectors, so the hard drive is OK. I found a similar thread here where the user had the exact same error, but it was caused by an update, whereas I have no clue abut what is the cause. I booted from a USB installation of build 2004 and went ahead to run the commands that were described in the accepted answer. But there was a problem. The Windows drive was not C: as it should be, but it was F: (the Windows installation media had the D: letter). C: was "System Reserved". I am running sfc /scannow with the offline directories set up, but it's stuck on "Beginning system scan" with the HDD activity on constantly for the last 25 minutes.(EDIT: SFC finished with no errors, it maybe was just not showing any progress.)

Is it possible that a virus changed my default drive letter? Or is it a corrupt system file?

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  • Sounds like the hard drive is failing to me. Copy your data off before it dies completely.
    – Moab
    Jul 5 '20 at 15:19
  • @Moab It is surely a possibility, but the weird part is that yesterday it did not show any signs of failure. Windows should have informed me with a pop-up as it usually does (I had a HDD fail on a previous Windows 7 installation and it did inform me with an error message saying "Your hard drive is likely to fail, back up your data".
    – na-no.
    Jul 5 '20 at 15:24
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    You don't always get a warning.
    – Moab
    Jul 5 '20 at 15:25
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    In rare cases the sata data cable goes bad, you might try replacing that first to see if it makes any difference.
    – Moab
    Jul 5 '20 at 15:47
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    "a BIOS message popped up saying that time and date were not set correctly."--this suggests the CMOS battery-backed settings have been lost, which happens when the battery expires. If that has happened, then your other BIOS settings will also have reverted to defaults. This could mean it's using the wrong disk info on boot, resulting in the issues you're seeing. Look for a battery on the motherboard (often the flat round CR2032) and replace it with a new one. Then edit and save your BIOS settings to fix any that are wrong.
    – Reg Edit
    Jul 5 '20 at 19:11
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I found the culprit. It was the CMOS battery. It must have died overnight, thus reverting the BIOS settings to the default ones. This explains the weird behavior I experienced when I first tried to boot the system.

The disk interface was reverted to "Intel RAID" without any notification whatsoever. After a painful 7 hour attempt trying to fix my system with every recovery tool I could find on the bootable USB drive that I have (including a System Restore with a point created a week ago), I thought I should dig deeper in the BIOS. I then proceeded to change "Intel RAID" to "AHCI", and lo and behold, my system booted up normally. It just took a long time because of the System Restore process (I had a mechanical HDD, you see...)

Huge thanks to @Moab and @RegEdit (in the comments above) for taking time to help me out. I really appreciate the community here.

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