MBR2GPT Conversion Failed Halfway What to Do and What Not to Do
A failed conversion feels like the worst outcome in computing. In practice the disk is usually still MBR and untouched, or fully GPT and only waiting for its firmware switch. Your job is to find out which — slowly.
✍️ Technical Anshu🪟 Windows 10 / 11⏱️ 6 minute read
🎥 Watch the full video — a clean run, so you can compare what your screen did differently
First, the honest framing: where the tool stopped decides everything, and no page can promise which state your disk is in. What this page can do is give you the calm sequence professionals use — establish the disk's actual state, then act on evidence. MBR2GPT validates first and writes its new layout as a late, deliberate step, so most interrupted runs leave the disk exactly as they found it. The dangerous moves are the panicked ones taken before checking.
Windows Recovery — Command Prompt, identifying the disk state (example)
X:\Sources> diskpart
DISKPART> list disk
Disk ### Status Size Free Dyn Gpt
-------- ------------- ------- ------- --- ---
Disk 0 Online 476 GB 0 B *
👆 Example layout. A star in the Gpt column means the GPT table is written; no star means the disk is still MBR — that one fact picks your route
1
Do Not Do These Things First
Do not run clean in diskpart, and do not format anything. Those commands finish the job of destroying a disk that is probably fine.
Do not re-run /convert repeatedly hoping for a different mood. If the cause is still there, the result will be too.
Do not keep power-cycling a machine that is mid-write. If a conversion is genuinely still running, let it end; if it has ended in an error, it has ended — proceed to diagnosis.
Do not "fix" it with a random third-party converter on a disk whose state you have not identified yet.
2
Case A — The Tool Errored, but Windows Still Boots
This is the common one, and the kind one. An error message with a prompt behind it usually means the tool stopped during its checking or preparation stages — the disk is still MBR and Windows never noticed anything.
Confirm in Disk Management: partition style still reads Master Boot Record (MBR).
Read the newest entries in setuperr.log and setupact.log in the Windows folder. The tool logs which check or step failed.
Fix that cause — a partition count, a space shortfall, a damaged BCD — then go back to /validate and only convert when validation passes cleanly.
Treat the failure as the validation system doing overtime, not as damage.
3
Case B — The PC Will Not Boot After the Run
Now identify the disk's state before touching anything. Boot a Windows USB, choose Repair your computer → Troubleshoot → Command Prompt, and inspect:
Run diskpart, then list disk. Look at the Gpt column for your disk.
Star present — the disk IS GPT now. The conversion likely completed and the "failure" is the firmware still set to Legacy. Enter firmware setup, switch boot mode to UEFI, and boot. This resolves the majority of won't-boot-after-conversion stories.
No star — the disk is still MBR. The conversion did not take hold; the problem is the ordinary Windows-won't-boot family. Use the recovery environment's Startup Repair, and restore your pre-conversion backup image if the boot configuration is damaged beyond a quick repair.
The disk does not appear at all, or reports a wrong size. Stop. That is a hardware-level conversation — cables, enclosure, a genuinely failing SSD — not a partition-table one.
4
The Power-Cut Scenario, Stated Plainly
If the power died during the write phase, the disk can be left between states, and no honest guide can walk that back step by step. This is the exact scenario the backup in the main guide exists for: restore the image, confirm the machine is healthy, and — if you still want GPT — run the whole sequence again on a stable power supply. A UPS or a laptop battery is not paranoia for partition work; it is the price of the job.
5
Before You Try Again
Know which case you were in (A or B) and what caused it, from the logs.
Run a disk health check — a conversion failing on a sick SSD will keep failing.
Re-validate until it passes, then convert once, cleanly.
Keep the backup until the converted machine has booted happily several times.
🔴 The rule that covers every version of this page: identify the disk's state first, act second. Every recovery disaster story starts with someone fixing a state they had only guessed at.
🎥 Watch the Complete Conversion in the Video
Watch the video's clean run once more before retrying — matching your screen against a known-good sequence is the fastest diagnosis there is.
The tool failed with an error. Is my disk corrupted now?
Usually not. When the tool stops on a failed check or an early error, the disk normally remains MBR and unchanged — verify the partition style in Disk Management, read setuperr.log for the reason, fix that cause, and validate again.
It converted but Windows will not boot. What is the first thing to check?
The firmware mode. Boot into setup and switch from Legacy/CSM to UEFI — a GPT system disk cannot boot the Legacy way, and this single setting resolves most post-conversion boot failures.
Power went out during conversion. What now?
Boot from a Windows USB and check list disk in diskpart. If the disk shows as GPT, switch the firmware to UEFI. If it is still MBR, repair the boot or restore your backup image. A disk caught truly mid-write is a restore-from-backup case — which is why the backup step exists.
This guide is based on the Technical Anshu MBR to GPT conversion video • Stuck on a step? Ask in the video comments — you will get a reply 🙏