Before converting to GPT it is important to know why and if you can convert. This article should help you make that decision.

MBR

MBR is standard method to present partitions to the bios so operating systems can load. The BIOS is hardcoded to read the first part of any hard drive, this is the MBR. In the MBR is stored the locations of the primary partitions. Each physical disk can have up to 4 primary partitions and within this you can create an extended partition which can hold up to a theoretical limit of 25 partitions (I think!).

Only primary partitions can be booted from and to boot from a partition, it must be set to active.

You can only have 1 active partition per physical disk so this limits the bootable partitions on a hard disk using MBR to 1. MBR is now known as a legacy method. MBR is limited to drives under 2.2TiB in size as it cannot address any larger than this.

In the bios there are instructions to load and execute certain files that are found in the root of the active partition and because these instructions are hardcoded in the bios it enables your OS to be loaded even after power down. Operating systems know this and place the appropriate files for the boot procedure in the root. If these files get corrupted or deleted the OS will fail to load.

GPT

GPT is the new way of addressing partitions on a drive. I believe Linux has been using this for a while and it is not limited to 2.2TiB drive sizes amongst other things.

GPT hard disk configurations also put a dummy MBR in place that reports the whole disk as ‘allocated’ (check this) to prevent Non-GPT aware programs corrupting the disk.

GPT Notes

• UEFI only works on 64-bit systems
• Secure boot will only work with UEFI enabled
• Secure boot only allows signed drivers to be loaded for the kernel. I know Microsoft has a certificate and there might be some other company certificates in there aswell.
• UEFI can be extended on the fly by loading files from a HDD or other boot media. This is how the Microsoft EFI partition works and its purpose.
• You don’t need the recovery partition to get GPT to work but it is handy.
• The MSR partition is not currently used but it might be in the future and it is installed in default Windows 10 installations.If It is is not present, your GPT Windows 10 will still load
• If you convert from a system disk that has always been an MBR, Windows will not boot via UEFI after converting to GPT. You need to add the EFI partition to allow booting.

GPT is not UEFI/EFI

Windows 10 needs a EFI partition to be able to boot windows that is located on a GPT hard disk.

The easiest way to clear this up is to describe the different technologies used by Windows 10 in this new setup.

• GPT is the new way partitions are addressed on the hard disk
• UEFI/EFI (User Extensible Framework Interface) is a method of extending a bios by adding drivers on the fly. There are several vectors to add these drivers.
• EFI Partition - This is where you can place drivers you want to be loaded by the bios. This is done on the fly at every boot up. It is by this mechanism that Windows 10 adds the drivers into its kernel so it can be booted from a GPT partition and also utilise Secure Boot

Questions

• Can you boot windows of a GPT partition without an EFI partition?
  
  I don’t think so because Windows uses the EFI partition to add extra driver(s) that sit in between windows and the Hard Disk which allows for the continued support of MBR on the same core code.
  
  
• Can you use a GPT partition for storage if your Windows installation is on a MBR system disk?
  
  I think the answer to this is yes. You just would not be able to boot of the GPT partition.