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Could you explain how Protective MBR works? As far I know, Protective MBR uses for backward compatibility with old software that was used for MBR. But how those programs see the disk? So, if you use old a software to manage GPT disk, that can only read MBR, this software will consider the whole disk like a one partition. Am I right?

asked Feb 4, 2023 at 16:07

2 Answers 2

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Legacy tools will simply 'see' a partition in legacy partition table (MBR) occupying the entire disk and as such the partition (hopefully) 'protects' the drive from new and overlapping partitions being created.

There's nothing else to it.

Legacy partition table with protective partition (0xEE):

enter image description here

answered Feb 4, 2023 at 16:53
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    I know this is the right answer... I tried to find the supporting documentation online but my Google-Fu is failing me on this one. Upvoted. Commented Feb 4, 2023 at 17:09
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But how those programs see the disk?

Legacy operating systems can see the space of the whole disk, for instance 4 TB, 8TB or even more.

Once they recognize the only partition scheme they are aware of, MBR, they are bound by the limitation of the MBR scheme which shows them that there is no space left anymore.

So, if you use old a software to manage GPT disk, that can only read MBR, this software will consider the whole disk like a one partition. Am I right?

Not necessarily. With a sector size of 512 byte they can guess that they can only address 2 TB in total although the disk has a total size of 4 TB p.e.

But they cannot do anything about it.

If they do not use sufficiently long integer variables legacy systems might fail when showing the total disk size showing you a size of

realsize mod 2^x

where x denotes the width of the integer variable.

answered Feb 5, 2023 at 21:51

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