By Cherry / Last Updated May 15, 2020

Can't partition unallocated space on 3TB hard drive

“I have a Seagate 3TB hard drive, and I install Windows 10 on it. However, I forgot to partition hard drive at the beginning of the Windows installation. I used Windows Disk Management to create two 1TB partitions, and I was trying to create a third partition with the left unallocated space. But, when I right click it, I found the “New Simple Volume” option is grayed out.

New Simple Volume

I recognized that MBR disk only supports up to 2TB, so I had a plan to convert MBR to GPT disk. Unfortunately, the option to change hard drive to GPT is also grayed out.

Convert to GPT Grayed

What should I do? Is there anyway that I can partition 3TB hard drive?”

Why you can't create a new volume with unallocated space on 3TB hard drive?

A MBR disk supports up to 2TB hard drive, which is the reason why your 3TB hard drive only shows 2TB, and why you can’t create a new volume with unallocated space on 3TB hard drive. No matter how big your hard drive is, you cannot use any space beyond 2TB. While GPT disk supports more than 2TB hard drive. If you want to partition your 3TB MBR disk totally, then you need to convert MBR to GPT disk before partitioning.

Windows Disk Management allows you to convert MBR to GPT disk, but it requires the disk must be an empty disk. Otherwise, it cannot be converted. Thus, you need to delete volume(s) you have created before converting. Or, you can use AOMEI Partition Assistant to convert MBR to GPT without losing data.

Note: GPT disk requires UEFI start mode. Before converting, you need to make sure that your motherboard supports UEFI partition table.

Partition 3TB hard drive with AOMEI Partition Assistant

AOMEI Partition Assistant is a great partition manager that can help you convert boot disk between MBR and GPT. It is an amazing software that you can use to partition your 3TB hard drive. Here is the specific guidance of how to create a new volume with unallocated space on 3TB hard drive.

Step1. Install AOMEI Partition Assistant Professional on your computer. Launch it. It will display you the basic environment of your disk(s). Here you can see your Seagate 3TB hard drive with two 1TB partitions, and unallocated space.

Main Interface

Step2.First, you need to convert the MBR disk to GPT disk. Here, right click the disk, select Convert to GPT Disk.

Convert to GPT

Step3. It will ask you if you are sure to convert the selected disk from MBR to GPT disk. Click OK.

Make Sure

As you installed system on the disk, it will give you a note that you should make sure if your motherboard supports EFI/UEFI boot. If not, your system will be unable to boot next time. If it supports, clickYes.


Step4. Then, you will back to the main interface, and you will see the disk has already became GPT disk. Click Apply to submit.

Apply Convert

Then, click Proceed to execute.

Convert MBR to GPT

Then, click Yes to confirm.


Wait till the proceed complete. Then, reboot your computer from EFI/UEFI firmware.


After the conversion, you can use AOMEI Partition Assistant to create a new volume with unallocated space on 3TB hard drive.

1>. Right click the unallocated space on 3TB hard drive, select Create Partition.

Create Partition

2>. Then, you can specify the partition size, drive letter, file system, partition type of your new partition. Click OK.

Edit Partition

3>. Then, click Apply at the toolbar to submit the mission.


AOMEI Partition Assistant is such a great tool for partitioning hard drive. It supports working with all Windows PC operating system including Windows 10/8/7. With the help of AOMEI Partition Assistant, you can partition 3TB hard drive simply like create a new volume with unallocated space on 3TB hard drive. Besides, it provides you many other features such as resize partition, split partition, and many more.