There are three common causes of the unmountable boot volume error, in which your computer experiences the blue screen of death (BSoD). It happens because the motherboard cannot access or mount your hard drive. The error means your hard drive may be damaged or corrupted. The error can also occur if the cable wires you used to connect the hard drive to the motherboard is not the standard 80-wire, 40-pin cable. The basic input/output settings (BIOS) may not also be properly configured, which forces it to access the faster protocol that transfers data to your hard drive.
Damaged Hard Drive
When your hard drive is damaged or corrupted, it becomes inaccessible and the operating system fails to load. The boot volume in the error refers to the partition in your hard drive, called a volume, that contains the operating system. Typically, this is the C: partition. Unmountable means inaccessible, which can be caused by several factors. The drivers, processes, applications that are loaded during startup may be corrupted or damaged. When any of the startup files fail to load, your system fails to load as well and crashes. You may have installed new applications or updated programs and files that are incompatible with the operating system. Viruses or malwares may have infected and corrupted your computer, spoofing or killing off the processes that need to run during startup. These are all possible causes of a damaged hard drive.
Wrong Connector Cable
You may have added or replaced your hard drive, disconnected your old one and connected a new one. You may have failed to notice that the cable you used was different from the old one. Some connectors are 40-wire, 40-pin cables, but the required one is an 80-wire, 40-pin cable. The wires are the ones that connect the motherboard to the pins. The pins are the endpoints of the wire that connect to your hard drive. When you boot your computer, the motherboard sends signals and data from its microchip to the hard drive telling it to load. When you use an incorrect wire, the signals sent are insufficient because the wire cannot handle the transmission, which causes your hard drive not to be mounted.
Improper BIOS Configuration
You may have inadvertently used a hard disk controller that forces your basic input/output settings (BIOS) to use it because it is faster. Not all motherboards support the ultra direct memory access (UDMA) hard drives. UDMA is a faster method computers use to access your hard drive, but some motherboards do not have the protocol or drivers to support it causing your system to crash. Reconfiguring your BIOS settings to use the Fail-Safe option during bootup fixes this problem.