Significant booting challenges on EFI systems when upgrading to Windows 8
Challenge #1 - Connecting your system disk 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 organization. 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 disk. 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 disk is the bootable one.
This means that you will need to update this record in order to get Windows back on rails.