How to Solve Unable to Convert a Disk to GPT in Windows Server?

May 18, 2018

This article demonstrates you how to convert a disk to GPT without data loss when the option convert to GPT disk grayed out in Windows Server 2003/2008/2012 (R2).

Unable to convert my disk to GPT in Windows Server 2012 R2

“I have Windows Server 2012 R2 installed on my machine and I want to convert my 3TB external hard drive to GPT, so that I can make full use of it. But I found the option of “Convert to GPT Disk”grayed out when I tried to change my hard drive from MBR to GPT partition style via Disk Management. Then, I tried to make conversion via Diskpart utility and only got the "Virtual Service error: The specified disk is not convertible. CDROMs and… "messageWindows Server unable to convert a disk to GPT via Disk Management or Diskpart, what should I do to change the partition table of my 3TB disk? Is there anyone can help me? Thanks a lot.”

Windows Server Unable to Convert a Disk to GPT via Disk Management

Unable to Convert to GPT Diskpart

Two ways that work actually to convert MBR to GPT in Windows Server

Compared with MBR partition style, GPT supports hard drive beyond 2 TB and allows you to create up to 128 primary partitions. To make unallocated space on hard drive over 2TB or to create more primary partitions on a hard drive, converting disk from MBR to GPT is necessary. If you encounter Windows Server unable to convert a disk to GPT via Disk Management or Diskpart utility, refer to the following text to realize how to convert a disk from MRB to GPT successfully.

Delete partitions and convert to GPT via Disk Management or Diskpart

In fact, Windows Server 2012 R2 is able to convert MBR to GPT via Disk Management if you have deleted all partitions on the hard drive at first. Do as the following steps to make conversion from MBR to GPT.

Backup volume via Windows Server Backup

(You can skip this if there is no data stored on the hard drive or there is no need to keep data saved on the disk,)

In order to avoid data loss, you can backup volume to local disk or removable device.

1. In the Search Box, type “windows server backup” to access Windows Server Backup utility.

PS.: If you have not used this function, you need to install (Server Manager\Features) it in advance.

2. Click Backup Once Wizard and follow it to execute backup settings.

Delete partitions and convert MBR to GPT via Disk Management or Diskpart

1. Press “Windows + R” simultaneously, type “diskmgmt.msc” and hit Enter key to open Disk Management.

2. Right click the partition that you need to delete and choose “Delete Volume...”.

3. After deleting all partitions on the disk that’ll be converted into GPT, you can right click the disk and choose “Convert to GPT Disk”

The whole conversion process will be completed within few seconds. After conversion, you can create partition with the unallocated space and restore backed up data.

Note: It is not allowed to convert removable media like USB flash drive or SD card to GPT partition style.

If you need to convert Windows Server system disk into GPT, you need to back up the whole disk at first. Then, boot your machine from Windows Server installation disc and press Shift + F10 to open the access Diskpart utility. In the Diskpart window, type the following commands in order and hit “Enter”key after every command:

  • list disk

  • select disk n, n refers the number of the disk that you need to convert

  • clean

  • convert gpt

Convert MBR GPT Diskpart

When you are prompted that Diskpart successfully converted the selected disk to GPT format, you can type “exit” and hit Enter key. 

Convert MBR disk to GPT without deleting your partitions

To convert data or system disk into GPT easier and faster, AOMEI Partition Assistant Server will be your great choice due to its intuitive interface and powerfulfunctions. It’s professional and comprehensive partition manager that is especially designed for Windows Server OS. It can help you to convert MBR to GPT without deleting partitions in Server 2003/2008/2012 and 2016. Moreover, it allows you to convert GPT disk to MBR without data loss as well. Look at the following steps to convert a disk to GPT in Windows Server.

Step 1. Ensure that your disk need converting is connected your machine well; Install and run AOMEI Partition Assistant Server.

Step 2. In the main interface, right click the disk that you’ll convert and select “Convert to GPT Disk”.

Convert to GPT Disk without Data Loss

Step 3. In the pop-up window, click “OK” to continue the operation.

Convert MBR GPT

Step 4. In the main interface, click “Apply” > “Proceed” to perform the operation.

Apply Proceed

PS.: If you converting the system disk into GPT, to boot from it, you need to change Legacy to UEFI.


When your disk owns partitions, Windows Server is unable to convert the disk to GPT directly using Disk Management or Diskpart utility. If you insist on doing so, you need to delete partitions on the disk at first. Or you can choose to use AOMEI Partition Assistant Server to convert a disk from MBR to GPT without deleting. You can also use it to clone Windows Server hard drive or move Windows Server from HDD to SSD.