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I recently purchased an external disk drive with 3 terabytes (2.72 TiB) of storage capacity.

Now according to Wikipedia's article on MBR, the maximum addressable storage space on a disk formatted using the MBR scheme is restricted to 2TB (1.81 TiB) so it stands to reason that high capacity hard disk drives (>2TB) must be formatted using GUID Partition Table (GPT) scheme to be able to use the entire capacity of the HDD.

Why is then the 3TB WD My Book external disk drive factory formatted with the MBR scheme and even more surprisingly, why does it show up on Windows as a 2.72 TiB drive when MBR can only address upto 1.81 TiB?

Has anyone else with a high-capacity Western Digital (or other brand) hard drive seen this before?

If it helps, the HDD is formatted using NTFS and the sector size is 4096 bytes. Also, here are a few screenshots:

3 TiB MBR WD My Book - 3TB


EDIT: After I posted my answer, I read this blog post from AnandTech which says:

Now LBAs under MBR partitions are addressed using 32-bit values, the maximum of which is 2^32 or 4294967296. Each LBA on a hard drive corresponds to a 512-byte sector value (even on 4K advance format drives, they still appear as 512-byte sector drives to the OS), so the largest partition you can have in a MBR partitioned drive is 4294967296 * 512-bytes or 2,199,023,255,552 bytes.

Hard drive manufacturers define 1TB as 1 trillion bytes. If we use that definition then the largest 32-bit MBR partition would be 2.199TB (2,199,023,255,552 bytes / 1,000,000,000,000). If we define 1TB as 1024^4 bytes (TiB) then the largest 32-bit MBR partition would be 2TiB (2,199,023,255,552 bytes / 1,099,511,627,776). Either way, with a 3TB drive there’s no way we’re getting a single 3TB partition using MBR.

But as you can see, the My Book I own has a single 3TB (2.72 TiB) partition and it's using MBR. The reason why this bothers me is because of this incident where a person claims to have encountered data integrity problems with the drive after transferring more than 2TB of data on the MBR formatted 3TB drive.

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    Is this an Advanced format (aka AF, aka 4K sector size) drive ? – Hennes Dec 14 '14 at 17:51
  • @Hennes Yes, I just found out about that right after I posted the question. That seems to be the reason why this is possible. – Vinayak Dec 14 '14 at 17:52
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    translate.google.com/… "Some external 3-TB-drives from Seagate (FreeAgent GoFlex) and Western Digital (My Book Essential) can be used thanks to another technical gimmick on XP at full capacity. Sign up through their USB or FireWire interface not with the usual 512-byte sectors but with 4K sectors to the system. In this way, the 2-TB-limitation of the MBR can be avoided." – bwDraco Jan 18 '15 at 20:26
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    I'm talking about 4Kn, where the drive is not emulating 512-byte sectors, but is exposing its actual 4K sector size to the system. – bwDraco Jan 18 '15 at 20:28
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    @DragonLord I just looked up 4Kn and I'm not sure it's a 4Kn HDD. I don't know what hard drive WD uses internally but I suspect it is a WD Green drive. – Vinayak Jan 18 '15 at 20:28
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Your drive uses Advanced Format 4Kn, where the drive exposes its native 4K sector size instead of emulating 512-byte sectors (512e). This ensures compatibility with devices that do not support GPT by extending the MBR limit to 17.6 TB (16 TiB), thereby avoiding the need to use GPT.

  • MBR permits a maximum of 232 sectors. This means that with conventional 512-byte sectors, the maximum supported disk size is 2.2 TB (2 TiB). However, with 4K sectors, the maximum disk size becomes 17.6 TB (16 TiB).

  • According to this article (via Google Translate, lightly edited), several external hard drives exceeding 2.2 TB in capacity use 4K native sectors with an MBR partition table for compatibility with Windows XP:

    Certain external Seagate (FreeAgent GoFlex) and Western Digital (My Book Essential) external 3-TB disks can also be used on XP at full capacity thanks to another technical twist. They do not [connect] to the system via their USB or FireWire interface [with the usual] 512-byte sectors, but rather with 4-KB sectors. This bypasses the 2TB limitation of the MBR. But even this trick has drawbacks: programs that access the disk low-level - such as formatting tools - may not work on the disks. [T]hey are [also] not suitable [as boot disks].

  • The manufacturer is taking advantage of the fact that this is an external hard drive, with an assumption that it will only be used for storage and not booting. However, this creates another set of compatibility problems, where programs that rely on 512-byte sector sizes may not work correctly:

    • Partitioning tools that do not support 4Kn drives will not function correctly with this drive.

    • Some database programs use direct disk I/O for performance and data integrity. These programs will not function correctly if they are not designed to handle 4K sectors.

  • The underlying internal hard drive may well be 512e—if this is the case, the enclosure electronics are exposing 4K sectors to the system for compatibility.

  • While the drive itself is probably indeed Advanced Format, I’m willing to bet it doesn’t really expose its native sector size. Instead, the USB-SATA bridge chip does that. – Daniel B Jan 18 '15 at 21:11
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    @DanielB: If the drive is 4Kn from the standpoint of the system, it's unlikely the drive is 512e with the USB-SATA bridge converting back to 4Kn. It's much simpler that the underlying drive is 4Kn, and it takes little more than firmware to do this. – bwDraco Jan 18 '15 at 21:14
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    It might be simpler, but there are tons of enclosures doing all sorts of dark magic to enable 3 TB partitions that also work on XP. – Daniel B Jan 18 '15 at 21:15
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    @DanielB I ran fsutil fsinfo sectorinfo <drive_letter:> and it reports the logical and physical bytes per sector to be 4096. I'm not going to take it apart and connect it to a SATA port and run the same command again anytime soon, but when and if I do, I'll report that here. – Vinayak Jan 18 '15 at 21:18
  • @Vinayak That’s a given, otherwise the MBR partition at this size would not be possible. WD doesn’t offer 4Kn drives though, so it’s unlikely the drive itself is 4Kn. – Daniel B Jan 18 '15 at 21:20
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I got in touch with a WD support technician and he told me that it was an anomaly and that I should convert the disk to GPT instead to make sure I don't encounter any accidental data loss in the future.

However, he was unsure about whether or not it would actually cause any data loss but recommended I convert the disk to GPT and he also let me know that mine was the first case of a MBR formatted high-capacity drive (>2TB) that he's encountered.

If you have encountered a similar situation before and know why this is or isn't an issue, please add your answer.

Disregard everything I wrote below. It may or may not be correct.


I think I found the answer. @Hennes and AIDA64 helped. It seems like Western Digital has come up with a solution to prevent the slow demise of MBR and it's called Advanced Format.

With this technology, WD has increased the block size of the HDD from 512 bytes to 4 kilobytes so Advanced Format drives can grow to 16 TB without hitting the 32-bit LBA size limit.

Stephen Foskett has explained this very well in his blog post.


From the blog:

Current hard disk addressing scheme tops out around 2.1 TB. There are two simple factors in this equation:

  1. There are 4,294,967,296 addressable blocks in a 32-bit “namespace”, and this is all most PCs can handle
  2. Each block is 512 bytes in size, and has been since dinosaurs sold the computers

If we multiply 4,294,967,296 times 512, we get 2,147,483,648 KB of capacity. In wacky base-10 storage industry speak, this is 2.15 TB of capacity.

The hard disk drive manufacturers have two roads forward, and they are not mutually-exclusive:

  1. Western Digital was first out with an Advanced Format drive late in 2009. This addresses the (archaic) block size limit, increasing it from 512 bytes to 4 kilobytes. With each address now eight times larger, we need one-eighth as many addresses for a given drive size. Advanced Format drives can grow to 16 TB without hitting the 32-bit LBA size limit.

  2. Seagate attacked the other variable, increasing the address space to the full 48 bits specified in the LBA standard. This gives an amazing 128 petabytes of capacity even with tiny 512 byte blocks.

  • I think I'd prefer Seagate's solution, with lots of tiny files @ 4k each they can take up a lot of space pretty fast – Xen2050 Dec 14 '14 at 18:44
  • @Xen2050 - What? The small files take the same amount of space no matter what. – Ramhound Dec 14 '14 at 18:51
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    @Ramhound Not when a bunch of <4k files are used on a drive with a block size of 4k, now they each take up 4k. – Xen2050 Dec 14 '14 at 18:58
  • You still get the same amount of data out of the storage device though. Think of it this way. Before a 4kb file would take multiple blocks not it only takes up one. – Ramhound Dec 14 '14 at 19:08
  • @Ramhound So, for a 4KB file, you use 4KB before and 4KB now—no difference. But for a 1B file, you use 512B before and 4096B now—a loss of 3584B. In fact, if we can expect file sizes to be uniformly distributed mod 512 bytes, which seems at least superficially reasonable, then we can expect to lose (4096B/2 − 512B/2) = 1792B per file on average. – wchargin Dec 15 '14 at 0:09
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I'm inclined to go with the blog rather than the WD support technician's reply. I have a couple of Toshiba Stor.e Canvio 3TB drives. Both are factory formatted using 'advanced format' with cluster and sector sizes of 4k.

Drives are used on a Samsung smart TV which does not recognize GPT partition (most multi media devices will only recognize MBR).

  • That is interesting. Have you used up more than 2 TiB of of storage on any of those HDDs? – Vinayak Jan 18 '15 at 20:16

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