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Various sources (e.g. 1, 2, 3, 4) claim that hard drives using the MBR partitioning scheme are limited to a maximum partition size of approximately 2 TB, and that you need GPT to support larger partitions.

However, I just discovered that one of our external HDDs uses the MBR scheme, but contains a single 3 TB partition.

disk management screenshot

How is that possible?

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According to the "The gen on the 2TiB disc size limit" FGA, the limit comes from MBR having a 32 bits wide field for a partition's sector count. With 512 byte sectors (the usual size), you can address only up to 512 × 232 bytes, or exactly 2 TiB.

Many disks these days have larger physical sectors of 4 KiB (called "Advanced Format" in various places); however, they almost always report each physical sector as eight logical 512-byte sectors, in order to remain compatible with all existing software (which might make incorrect assumptions about sector size always being 512 b). So they are still affected by MBR's limitations.

But I'm guessing (although the screenshot doesn't show) that your disk probably reports the logical sector size as 4 KiB as well, and since the MBR can address up to 232 sectors, you now get a partition limit of 4096 × 232 = 16 TiB.

(This is not to be interpreted as "MBR is okay again". No, MBR with its primary/logical partitions is still as horrible as it was before.)

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Well guessed! fsutil fsinfo ntfsinfo z: shows Bytes Per Sector: 4096. –  Heinzi Mar 7 '13 at 17:09
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Some external disks -- even some containing disk drives with 512-byte physical sectors -- report that they have 4096-byte sectors. To the best of my knowledge, no commonly-available internal hard disks yet report a 4096-byte sector size, although I'd be surprised if such things didn't start showing up eventually. –  Rod Smith Mar 8 '13 at 2:41
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