When you add a new disk into your machine, you should initialize it before it can be used for storing and retrieving. In other words, a new added disk must be registered in your operating system by initializing it. This feature allows you to initialize a disk into MBR or GPT mode.
To Initialize Disk
Step 1: Launch AOMEI Partition Assistant and right click on the raw disk which you want to initialize, let's take Disk 2 as an example. Choose "Initialize Disk" in the pop-up menu.
Step 2: In the pop up window, there will be two options for you to choose. You can initialize the disk to MBR or GPT style. Click "OK" to continue.
Step 3: Click "Apply" to wait for process to complete.
The Initialize Disk option will be grayed out if the disk has already been initialized.
As for detailed info about difference between MBR disk and GPT disk, please refer to this article: How to Convert GPT to MBR or Convert MBR to GPT Disk?