UEFI Partition Restore to Cloud VMs (EC2, Azure)

This chapter covers the following topics:

UEFI Boot Explained

UEFI is a specification that defines a modernized model for the interface between computer operating systems and platform firmware during the boot or start-up.

In many ways, UEFI serves as a software-driven, bare-bones operating system and like BIOS, UEFI is responsible for initializing the hardware of a device before passing control of the hardware to the operating system. The latest platforms support both UEFI and legacy BIOS boot in order to ease the transition to UEFI and accommodate older operating systems that don't have built-in UEFI support.

The UEFI specification offers advanced features over BIOS such as:

  • Secure boot
  • Low-level cryptography
  • Network authentication
  • Universal graphics drivers

The Secure Boot functionality in UEFI provides the basis for the Microsoft Secure Boot feature in Windows 8 that enables the OS to detect rootkits and similar malware attacks.

Restore GPT/UEFI Machines to EC2 VMs

Amazon Web Services states the following limitations concerning UEFI boot partitions:

  • UEFI/EFI boot partitions are supported only for Windows boot volumes with VHDX as the image format. Otherwise, a VM's boot volume must use Master Boot Record (MBR) partitions. In either case, boot volume cannot exceed 2 TiB (uncompressed) due to MBR limitations. Additional non-bootable volumes may use GUID Partition Table (GPT) partitioning but cannot be bigger than 16 TiB. If you use VMIE APIs (instead of AWS Server Migration Service), you will have to construct a manifest file for disks larger than 4TB
  • When AWS detects a Windows GPT boot volume with a UEFI boot partition, it converts it on-the-fly to an MBR boot volume with a BIOS boot partition. This is because EC2 does not directly support GPT boot volumes on Windows instances.

In practice, restore jobs of UEFI boot partitions to EC2 are often terminated with errors. Thus, it is not recommended to select EC2 instances for restore jobs containing UEFI partitions and use the Convert to MBR feature instead. You can try the restore to EC2 at your own risk.

Read more about GPT to MBR conversion in the Convert GPT Disks to MBR chapter

To learn more about EC2 limitations, refer to VM Import/Export Requirements article at docs.aws.amazon.com