By Cherry / Last Updated January 14, 2021

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. So why and how can I partition this 3TB hard drive?

New Simple Volume On 3tb

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

So, why you cannot create a new partition with the unallocated space on 3TB disk? That’s something wrong with the partition style the disk uses. If you check the partition style, you will find it is MBR. This partition style only supports up to 2TB hard drive. Namely, on a 3TB MBR-based hard drive, only the first 2TB hard drive is usable, the rest 1TB disk space will be shown unallocated in Disk Management, which be the main reason why many users find their 3TB hard drive only shows 2TB.

How to partition 3TB hard drive to use its full size?

Now we know it is the MBR partition style that makes you unable to create a new partition with unallocated space on the 3TB hard drive. So, we need to change the partition style to another one: GPT. It is a newer partition style, breaks out the MBR disk size limit and can support up to 9.4ZB. Check the following table to get more information between those 2 disk styles.


Then, here comes the problem: how to convert 3TB hard drive from MBR to GPT? Disk Management provides the conversion feature, but when you right-click the 3TB hard drive and try to select the option “Convert to GPT Disk”, you will find it is grayed out. That’s because this tool can only convert an empty disk (without any partitions on) to GPT/MBR. Therefore, you have to delete all partitions along with data beforehand.

Convert To Gpt Grayed

Best 3TB hard drive partition software

What if you don’t want to delete volumes you have created on the 3TB hard drive before converting it to GPT? Lucikly, there is another option: apply the AOMEI Partition Assistant Professional software. It can convert disk to MBR or GPT without deleting any existing partitions on the disk, so the data wil be safe during the conversion. Next we will show how to convert a 3TB data disk as an example.

Note: GPT disk requires UEFI start mode. Before converting, you need to make sure that your motherboard supports UEFI partition table if the 3TB MBR disk is the boot disk.

Step 1. Install AOMEI Partition Assistant Professional on your computer (You can download its demo version for a trial first). Launch it. It will display you the basic environment of your disk(s). Right click the 3TB disk and select "Convert to GPT Disk".

Convert 3tb Mbr Disk To Gpt

Step 2. It will ask you if you are sure to convert the selected disk from MBR to GPT disk. Click OK.

Ok 3tb Disk

✍If 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, click Yes.

Step 3. Then, you will back to the main interface, and you will see the disk has already became GPT disk. Click "Apply" to submit.

Apply 3tb

Wait till the proceed complete. 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 On 3tb

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

Partition Detail

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



AOMEI Partition Assistant is such a great tool for users to partition 3TB hard drive and create a new volume with unallocated space on 3TB disk. It supports working with all Windows PC operating system including Windows 10/8/7. Besides, it provides you many other features such as resize partition, split partition, merge partitions, transfer OS to SSD and many more. For Windows Server users, the AOMEI Partition Assistant Server is a great tool.