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In simple terms = what is UEFI?

What is UEFI?

When I have had a PC that won't boot in the past, I've been able to take its hard drive, connect it to a working PC, and access the data on the drive.

I have had a PC that I couldn't do that with.  It had UEFI enabled and Windows 8 installed.

Was I just unlucky with something else, or does UEFI prevent a PC from accessing another PC's hard drive in the way I describe?  I got a message with words to the effect "That drive isn't formatted, do you want to format it now?".
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RedLondon
Asked:
RedLondon
3 Solutions
 
Patrick BogersDatacenter platform engineer LindowsCommented:
Unified Extensible Firmware Interface (UEFI) is the succesor for what we know as the old BIOS.
Big advantages are it is not bound to the 1MB limit BIOS's were and it is not glued to the x86 architecture.

When you deploy Windows® to a UEFI-based PC, you must format the hard drive that includes the Windows partition by using a GUID partition table (GPT) file system which is not fully compatible with systems running old BIOS. (most Intel board support EFI booting but may require a bios update)
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comfortjeaniusCommented:
BIOS works by reading the first sector of the hard drive which has the next device’s address to initialize or code to execute. It also selects the boot device that needs to be initialized for starting the operating system.

UEFI stores all the information about initialization and startup in an .efi file instead of the firmware. This file is stored on the hard drive inside a special partition called EFI System Partition (ESP).

UEFI will eventually replace BIOS in the near future.  The nuances that it provides are breaking out of size limitations, speed and performance, and Security.

UEFI can allow only authentic drivers and services to load at boot time, making sure that no malware can be loaded at computer start-up.  The secure boot feature is also one of the reason why it is more difficult to install another operating system on a Windows machine.
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Gary CaseRetiredCommented:
Apples and Oranges :-)

Whether your system has a traditional BIOS, or the new Unified Extensible Firmware Interface (UEFI) by itself has NO impact on the issue you asked about (i.e. connecting a drive to another system).

Technically, a UEFI system does not have a "BIOS" => the BIOS interface has been replaced by the UEFI.     But in popular usage "BIOS" refers to BOTH the classic BIOS interface and a UEFI ... they're often referred to as a "Legacy BIOS" or a "UEFI BIOS".    

However, with regard to the issue you've asked about.   No, whether a system has a UEFI interface or a BIOS does not make any difference in whether or not you can attach a drive from another system and access it.

What's most likely happening in the case you noted is that the system has a TPM module (Trusted Platform Module) and that the disk is encrypted, so it can only be accessed with the hardware-based security key.    Assuming that's the case, you need to save the key on a USB drive along with a small executable that allows accessing the drive on other systems.    Then, to access the drive, you'll need to install the executable on the system you want to "see" it on, and set the key ... then the system will be able to access the drive with no problem (although you may still need to "take ownership" of the file system to access the files).

Many Windows 8 systems with Secure Boot (which Microsoft requires for all OEM installs) have TPM security as well.    This will undoubtedly become more of an issue as folks start to have problems with the hard drives and want to access them on other systems for recovery.    It's a good idea to export the security key and the necessary .exe file NOW on such systems, so you have them available should that ever become necessary.
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