The 'Reboot and Select Proper Boot Device' error is one of the most frustrating issues a PC user can face. It usually appears during the startup process, preventing Windows from loading. This error indicates that the BIOS/UEFI cannot find the storage device (SSD or HDD) that contains your operating system, or it doesn't recognize it as a bootable source. Whether you just installed a new drive or the error appeared out of nowhere, this guide will help you fix it.

1. Check Your Physical Hard Drive Connections

The most common cause of this error is a loose or faulty cable. If the motherboard cannot communicate with your SSD or HDD, it simply won't boot.

• Power down your computer and unplug it from the wall.
• Open your PC case (or the storage compartment on a laptop).
• Locate your SATA cables (for 2.5-inch SSDs/HDDs) or ensure your M.2 NVMe SSD is firmly seated in its slot.
• Unplug the cables and plug them back in firmly. If you are on a desktop, try switching to a different SATA port on the motherboard.
• Ensure the power cable from the PSU is also securely connected to the drive.

2. Verify the Boot Order in BIOS/UEFI

Sometimes the BIOS resets or changes the boot priority, especially after a power surge or a CMOS battery failure. You need to ensure your primary drive is set as the #1 Boot Priority.

• Restart your computer and repeatedly tap the Delete, F2, F10, or F12 key (depending on your motherboard brand) to enter the BIOS.
• Navigate to the Boot tab or section.
• Look for the Boot Priority List or Boot Option #1.
• Make sure your Windows Boot Manager or the specific SSD/HDD where Windows is installed is selected as the first option.
• Press F10 to Save and Exit.

3. Toggle Legacy Boot and UEFI Modes

If you recently moved an old hard drive to a new computer, or changed BIOS settings, the Boot Mode might be the culprit. Windows installed on older systems often uses Legacy (CSM), while modern installs use UEFI.

• Enter your BIOS/UEFI settings again.
• Search for Boot Mode or CSM (Compatibility Support Module).
• If it is set to UEFI, try switching it to Legacy/CSM (or vice versa).
• Disable Secure Boot temporarily if you are trying to boot in Legacy mode.
• Save changes and restart to see if the drive is recognized.

4. Use Windows Startup Repair

If the hardware is connected and the boot order is correct, the Master Boot Record (MBR) or Boot Configuration Data (BCD) on your drive might be corrupted.

• Insert a Windows Installation Media (USB flash drive) and boot from it.
• Select your language and click Next, then click Repair your computer in the bottom left corner.
• Go to Troubleshoot > Advanced Options > Startup Repair.
• Windows will scan your drive and attempt to fix any corrupted boot files automatically.

5. Check for Hard Drive Failure

If none of the above steps work and the drive does not even show up in the BIOS list, the hardware itself may have failed. To confirm this:

• Listen for clicking or whirring sounds coming from a mechanical HDD.
• Try connecting the drive to another computer using a SATA-to-USB adapter. If the other computer cannot see the drive, it likely needs to be replaced.
• If the drive is detected on another PC, back up your data immediately and perform a Clean Install of Windows on the drive after re-formatting it.

💡 Pro Tip: Keep your software updated to avoid these issues in the future.