Windows: Convert Legacy Installation to UEFI

Warning: this guide requires basic knowledge of the command line.
Warning: there is a very slim chance that your data may be lost during this process. While it is very unlikely this may happen, we still recommend that you back up your data before performing these steps. Cyberdragon takes no responsibility for data loss.

If you are not comfortable proceeding with this guide, please feel free to reach out to us and we will be more than happy to help.

The following tools may be helpful for this guide:

  • A USB stick with a copy of the Windows installer.

As newer computers no longer support legacy boot modes, often times a conversion to UEFI is required when upgrading to a new computer. Or perhaps, it's simply time to convert for improved security. Whatever the reason is, it is luckily quite easy to convert from legacy boot to UEFI boot, using a built-in Windows tool.

First, you will need to get to a command line within recovery mode. You can do this two different ways:

From Within Windows

  1. Go to Start → Power
  2. Press and hold Shift and click Shut Down
  3. Troubleshooting → Advanced Options → Command Prompt

Using a Windows Installer USB

  1. Boot into your Windows Installer USB (see Boot + BIOS Keys)
  2. Press Next → Repair this computer (or press Alt-N → Alt-R)
  3. Troubleshooting → Advanced Options → Command Prompt

Once you have opened a command line, run the following:

diskpart
list disk # Note the disk number of the disk you are trying to convert
exit
mbr2gpt /validate /disk:<disk-number>
mbr2gpt /convert /allowFullOS /disk:<disk-number>

Note: certain Dell computers may include an OEM recovery partition which will confuse the conversion program and produce an error. If the Windows installation was performed on a Dell system and the command produces error code 0x00000000, use the following command instead:

mbr2gpt /convert /allowFullOS /map:222=ebd0a0a2-b9e5-4433-87c0-68b6b72699c7