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Windows 8 upgrade: Booting issues on EFI systems

10 Jan 2013  | Konstantin Komarov

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For upcoming new computers, laptops and servers, the new Windows 8 and Server 2012 will be the base operating systems. Unified Extensible Firmware Interface (UEFI) is the recommended platform for all of them. In spite of all of its advantages, many users are still not familiar with UEFI. There are known issues which may arise when you decide to manage EFI boot options or migrate to an EFI-based computer from another one. In most of these cases, your OS will simply not boot.

Challenge #1
Connecting your system disc to another MB port and Windows is not available!

What didn't used to be a problem on old BIOS-based computers is a major issue on new EFI-based ones. Let's see why this happens.

Most modern EFI implementations have strict rules for internal bootable device organisation. An EFI boot entry for Windows, that is Windows Boot Manager, also has its own restrictions and peculiarities which make all operations with bootable devices and volumes on EFI-powered machines with Windows even more complicated.

Let's have a look on how Windows Boot Manager in UEFI is linked to a specific device.

Windows Boot Manager contains several records: a bootable GPT drive ID and a device number. They are used by UEFI to resolve which device to use for OS booting.

So here is the problem: Assume that you have disconnected a bootable GPT drive with Windows from one SATA port on your motherboard and connected it to another one. You will get the following configuration:

As you may see, Windows Boot Manager now has an incorrect device number record. Actually, it simply points to your previous SATA port. The drive ID is correct, as it is unique for every connected storage device. It is possible that Windows will not boot until you connect the drive back to the original port or change the device number record in the Windows Boot Manager entry.

Challenge #2
You cannot boot Windows copy on EFI-based computer

The second challenge is replacing a bootable GPT HDD with Windows with larger or faster disc. Your migrated/copied OS will be unbootable due to incorrect links in UEFI.

Even if you replace your source drive with your target drive, your system will not boot because of an incorrect drive ID record in Windows Boot Manager. Instead, it will still be indicating that your original source disc is the bootable one.

This means that you will need to update this record in order to get Windows back on rails.

Challenge #3
Windows dual-booting is more complicated

By default, there can be only one Windows Boot Manager in EFI NVRAM. In such case, dual-booting will only be possible if the boot files for all Windows versions installed in EFI mode are stored on a single EFI bootable system partition called an ESP or listed in a single BCD store. So if something went wrong with the main bootable hard disc where all the boot data is stored, then all dependent Windows versions will become unbootable.


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