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UEFI GPT vs MBR

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AlbinoRhino View Drop Down
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    Posted: 06 May 2017 at 3:16am
Is there a benefit one way or the other when installing the OS in the UEFI vs MBR modes?

OS resides on its own M.2 drive with a 1TB HDD for everything else.

Should the two drives be partitioned the same way or does it matter?
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wardog View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote wardog Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2017 at 6:55am
UEFI OS drive FTW


Your storage drive either way if it already holds data
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AlbinoRhino View Drop Down
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Originally posted by wardog wardog wrote:

UEFI OS drive FTW


Your storage drive either way if it already holds data


The storage drive is blank. I guess it would just be easier to have them both GPT. Is there an option in Windows to GPT that drive or is it also done via the UEFI?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote parsec Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2017 at 10:54am
Originally posted by AlbinoRhino AlbinoRhino wrote:

Originally posted by wardog wardog wrote:

UEFI OS drive FTW


Your storage drive either way if it already holds data


The storage drive is blank. I guess it would just be easier to have them both GPT. Is there an option in Windows to GPT that drive or is it also done via the UEFI?


If the drive is blank and not formatted, you can initialize it as GPT in Windows Disk Management.

If you know how to use the Diskpart command from the Windows command line window, or Power Shell, you can run the Diskpart clean command on the drive and put it into an uninitialized state. Be sure it has no data you care about on it, you said it was empty so no problem.

Then start Disk Management and the uninitialized drive will be detected. It should automatically open this dialog box that allows you to select MBR or GPT partitioning:



Be sure that GPT is selected as shown above. You will then still need to fully format the drive in Disk Management by right clicking on the unallocated space of the drive and select New Simple Volume.

You can also use the Diskpart command, convert GPT, to change an existing MBR partitioned disk to GPT. The drive must be empty and have no partitions or volumes on it.

Why use GPT partitioning? Not easy to explain completely. The main reason to use it is when using high capacity HDDs, over 2TB in size. MBR partitioning which uses 32 bit LBA addressing, can only address up to 2TB. GPT partitioning uses 64 bit LBA addressing, and can address 9.4 ZB (Zeta-Bytes, 10 to the 21 power bytes).

GPT stands for GUID Partition Table which is used in the UEFI standard. NVMe SSD installed in UEFI booting mode are GPT formatted by the Windows installer. They then use the Windows EFI bootloader rather than the Legacy MBR bootloader. That allows you to disable the CSM (Compatibility Support Module) in the UEFI/BIOS, which when enabled runs the UEFI firmware in BIOS emulated mode. Running with CSM disabled allows the UEFI firmware to operate as it was designed to, 64 bit addressing and full access to DRAM memory. That in comparison to BIOS firmware, with 16 bit addressing, and access to all of 1MB of DRAM memory.

Running without the CSM should make POST and booting the OS faster. In practice, the difference is not that great, a few seconds difference overall. You can use the Ultra Fast setting of the Fast Boot option if CSM is disabled, which does shorten POST time more on most systems. You must have installed Windows with CSM disabled in order to use the Ultra Fast setting. Your video source must also support the UEFI GOP video protocol, or you'll get a black screen. Modern video cards should support GOP in their VBIOS, and Intel's iGPU VBIOS does since Sandy Bridge. Not sure about AMD.




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