Bluescreen of Death During Startup

Windows may start loading up, but then suddenly stop, showing a Bluescreen of Death (BSOD). The error messages onscreen may or may not give a sound indication of the problem. These errors are deemed serious because access to a working operating system is limited because Windows will not start normally.

Unbootable disk / Inaccessible Boot Device
The disk itself may not unbootable by Windows, even though, the disk itself is mechanically sound. A common symptom is the infamous, "Inaccessible Boot Device" message that appears in the BSOD. Generally, this symptom results from corrupted, uninstalled or incorrectly installed disk drivers for the disk controller. For example, when booting over the iSCSI protocol, or from a RAID disk array, special drivers need to be installed to prevent this error. In households, such configuration is unlikely. However, it may still occur if the BIOS is using incorrect settings to access the disk (such as trying to access it in AHCI mode when Windows was not initially installed in that mode). BIOS settings can be changed without deliberate user intervention such as a power failure. Computers have a button battery installed that retains the settings for the computer in case of power blackout. However, if the battery is faulty, the settings for the BIOS may be lost. Consequently, when the computer next boots, it may access the disk in an incorrect manner, resulting in an inaccessible boot device error.

Faulting Disk
BIOS misconfigurations are not the only cause for the inaccessible boot device BSOD error. Such an error may also occur if the disk itself is faulty. When a hard disk is faulty, it may not fail in a dramatic way. Some data may still be readable, depending on the fault. However, the damage may be severe enough to prevent Windows from loading. Generally, Windows tries to repair itself to no avail because it cannot physically repair damage to the hard drive unit itself. Damaged hard drive units tend to become very hot quickly and may produce overheating symptoms. Due to getting too hot, they may shutdown to prevent physical damage, which results in Windows being unable to access the disk. Ideally, if the disk is 3-5 years old and is performing loudly, it ought to be replaced even if Windows is loading successfully.

Filesystem corruption
Filesystem corruption occurs when the first 446 bytes of the MBR is corrupted. Filesystem corruption is not file corruption. The files themselves might be sound, but how Windows indexes those files is corrupt. A corrupted filesystem confuses Windows because it can no longer reliably determine which files are on the disk. The system may report corrupted or missing files, even when no corruption to the said files is present. Filesystem corruption is common when a computer is shutdown incorrectly, or may be a symptom of a failing hard disk. Although filesystem corruption occurs, the NTFS filesystem which modern Windows uses has mechanisms to recover from corruption. Typically, Windows includes a free chkdsk tool that can be used to scan for filesystem corruption and, in most cases, successfully repair the damage. Windows startup repair may automate this step automatically, but Windows does not always detect the corruption, so it is necessary to start the tool manually from a command prompt.

When Windows enters recovery mode, start a command prompt and then type:

 chkdsk c: /f

You should replace c: with the drive letter where Windows is installed. It tends to be D or E under a recovery environment. The tool will automatically repair the damage if any is detected. Once the repair is complete, you can reboot the system and try agin. BSODs themselves may cause filesystem corruption by halting the operating system before all of the data has been written to the filesystem. Thus, it is vital for journalling to be enabled on NTFS volume, though, it slows performance. Significant and irreversible corruption may result if journalling is haphazardly disabled. Journalling is enabled by default and no action is necessary to turn it on.