Boot error: 'An operating system wasn't found' after changing BIOS boot mode

The problem: After switching between UEFI and Legacy/CSM, firmware no longer finds the installed OS.

Symptoms: Boot message: OS not found/no bootable device after BIOS change; disk still detected.

Likely cause: OS installed in UEFI/GPT but firmware set to Legacy, or OS installed in Legacy/MBR but firmware set to UEFI-only.

Category: Boot & Startup · Difficulty: Moderate · Windows: Both

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