UEFI GPT vs Legacy CSM MBR Partition Table Boot Mode Mismatch
Fix boot failure where cannot detect a bootable drive due to mismatch between UEFI pure mode (requiring GPT) and Legacy CSM mode (requiring MBR).
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Common Symptoms & Tells
- boot menu reports: No bootable device found
- Storage drive appears in hardware list but not in boot priority list
- PC repeatedly drops directly into setup after disabling CSM or enabling Secure Boot
Interactive Fix Checklist
0 of 4 steps completedCheck disk partition table style in WinRE
Boot from Windows install media, press Shift+F10 to open command prompt, type diskpart, then list disk. If an asterisk (*) appears in the GPT column for your disk, it is GPT; if blank, it is MBR.
Enable CSM in BIOS for MBR disks
If your drive uses MBR partition table, enter setup, find Compatibility Support Module (CSM), set to Enabled, and set Boot Device Control to UEFI and Legacy OPROM.
Convert MBR disk to GPT without data loss
In Windows Recovery command prompt, run: mbr2gpt /validate /disk:0 /allowFullOS. If validation succeeds, run: mbr2gpt /convert /disk:0 /allowFullOS. This creates an EFI system partition and converts partition table to GPT.
Switch BIOS to pure UEFI mode
After successful conversion, re-enter , disable CSM, enable UEFI boot mode and Secure Boot, select Windows Boot Manager as primary boot device, and reboot.
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