When a newly built computer successfully passes POST, it indicates that the CPU, RAM, motherboard, video output, and other essential components are functioning at a hardware level. POST will typically fail or display error codes if there are problems with the CPU, RAM, GPU, or power delivery. Therefore, the fact that POST succeeds allows us to eliminate those components from suspicion.

The issue arises after POST, when the system attempts to load an operating system. For the OS to load, the BIOS/UEFI must detect a working storage device—such as an HDD, SSD, or NVMe drive—containing a bootable OS. If the computer cannot locate or read the drive, the system will halt at a boot error, such as “No boot device found,” “Insert boot media,” or similar messages. This is consistent with a failed or improperly connected HDD.

A failed HDD prevents access to boot files, even though the rest of the system hardware functions normally. Audio cards are irrelevant to booting. A failed CPU would cause a POST failure, not an OS loading issue. A faulty power supply would affect system stability during POST, not specifically OS boot failure unless the system is not powering at all.

Thus, the failed component is the HDD.