The Symptom
When you connect a disk larger than 2TB on your computer, the hard drive only shows
2TB in Disk Management and the option New Simple Volume is grayed out. While
creating a new partition on a large disk with third party software, you may receive
a message that the disk space beyond 2 TB will be unusable or something like
that.

Why the disk space beyond 2TB will be unusable?
There are two different partition styles, one is MBR (Master Boot Record), and the other is GPT (GUID Partition Table). While you initialize a hard drive, Windows will ask you to choose the partition style. An MBR disk supports maximum volume for 2 TB
(Terabytes) and holds at most 4 primary partitions. Therefore, you can't create a new volume on unallocated space beyond 2 TB on an MBR disk.
However, GPT disk
partition style supports maximum volume for 18 EB (Exabytes) and the limitation for
partition size depends on your operating system (Windows allows up to 128 partitions
on a GPT disk. Since the problem is caused by the limitations of the partition
style, you can convert MBR disk to GPT disk to solve it.
Solution 1: convert to GPT with Disk Management
Windows has its built-in partition tool Disk Management that can convert a blank MBR
disk (shown as unallocated space) to GPT disk. To do this, you have to delete
all the volumes. It is recommended to backup
disk before the conversion.
1. Open Disk Management. Then right click the partition on the disk your want to
covert and select Delete Volume.

2. You can see the whole disk shown as two separate unallocated spaces. Right click
the disk and select Convert to GPT disk.

3. After the conversion, you will be able to use the whole disk.
Solution 2: convert to GPT without data loss
If you want to convert to GPT disk without losing data, you can use third-party
software AOMEI Partition Assistant. It is available for both PC and Server users in many different versions. It supports Windows 10/8.1/8/7/XP/Vista and Windows Server
2012/2008R2/2008/2003. If you want to convert the non-system disk, you can free download the Standard version for PC or Lite version for Windows Server. If you want to convert system disk to
GPT, you need to upgrade to the Professional version for
PC or Server version for Windows Server.
1. Install and launch AOMEI Partition Assistant.

2. Right-click the disk and select Convert to GPT disk.

3. Click Apply to run the operation. Then you are able to create a new partition with the unallocated space.

Note: if you want to convert system disk to GPT disk, make sure your motherboard support UEFI Boot Mode, and enable UEFI boot mode after the conversion. The Standard
(free) edition is able to convert non-system disk to GPT disk without data loss. For
system disk, you can try AOMEI Partition Assistant
Professional for PC or AOMEI Partition Assistant
Server for Server.
Having solved the problem that the disk space beyond 2tb will be unusable, AOMEI
Partition Assistant can also be useful to many other matters as it provides multiple
features, such as Migrate OS to SSD,
Disk Copy, NTFS to FAT32 Converter, Windows to Go Creator, and
more.