A user tries to turn on a computer but gets the following error message: Bootable...

The most likely reason for this error is A. The HDD is malfunctioning.

A bootable device not found error usually indicates that the system cannot access or detect the hard disk that contains the operating system files. This could be due to a physical or logical damage to the hard disk, such as bad sectors, corrupted partitions, or faulty cables. According to the web search results, some possible causes and solutions for this error are:

Hard drive connection issue: Check the cabling and power supply to the hard drive and make sure they are properly connected12.

Corrupt hard drive partitions: Use a Windows installation media to scan and repair the corrupt system files using the System File Checker (SFC) tool1.

Damaged hard drive: Replace the hard drive with a new one and reinstall the operating system2.

Incorrect boot sequence in the BIOS: Enter the BIOS or UEFI settings menu and change the boot order to make sure that the system drive is in the first position13.

Damaged MBR (Master Boot Record): Use a Windows installation media to repair the Windows bootloader using the Diskpart tool1.