The 2TB Barrier – What Does it Mean for You?

Throughout last year I made a couple of mentions of the 2TB (two terabyte) barrier for physical storage in modern computers but didn’t take the opportunity to flesh out exactly what it means for the regular consumer out there. In this article I am going to explain why this is important and what you need to keep in mind when using or purchasing drives over 2TB in size.

It’s been a little while since we have seen hard drives break the symbolic 1TB barrier (much like the 1GB barrier back in the early to mid 1990s) however that symbolism comes awfully close to a technical barrier sitting at just twice that capacity at 2TB. The reason for this is the MBR (Master Boot Record) partitioning scheme in use in a significant number of new computers (not to mention older computers) and its limitation of 32-bit values. The maximum size addressable with 32-bit values is just over four billion sectors (i.e. 2 ^ 32 = 4,294,967,296 sectors).

Since hard drives (including the new 4K hard drives which have physical four kilobyte sectors but pretend to have 512 byte sectors) present themselves with 512 byte sectors this translates to a maximum size of 2TB:

  • 4,294,967,296 sectors * 512 bytes per sector = 2,199,023,255,552 bytes
  • 2,199,023,255,552 bytes / 1,024 bytes per kilobyte= 2,147,483,648 kilobytes
  • 2,147,483,648 / 1,024 kilobytes per megabyte = 2,097,152 megabytes
  • 2,097,152 / 1,024 megabytes per gigabyte = 2,048 gigabytes
  • 2,048 / 1,024 gigabytes per terabyte = 2 terabytes

… for a single partition or group of partitions on a given hard drive (or on a RAID setup for enthusiasts).

Quite clearly, with no further room to grow, the days of MBR are numbered.

The solution to push past the 2TB barrier is something called GPT (GUID Partition Table where GUID stands for Globally Unique IDentifier). GPT uses 64-bit addressing which translates to a maximum size of 8ZB (that’s zettabytes) using over 18 quintillion sectors (i.e. 2 ^ 64 = 18,446,744,073,709,551,616 sectors):

  • 18,446,744,073,709,551,616 sectors * 512 bytes per sector = 9,444,732,965,739,290,427,392 bytes
  • 9,444,732,965,739,290,427,392 bytes / 1,024 bytes per kilobyte = 9,223,372,036,854,775,808 kilobytes
  • 9,223,372,036,854,775,808 kilobytes / 1,024 kilobytes per megabyte = 9,007,199,254,740,992 megabytes
  • 9,007,199,254,740,992 megabytes / 1,024 megabytes per gigabyte = 8,796,093,022,208 gigabytes
  • 8,796,093,022,208 gigabytes /1,024 gigabytes per terabyte =  8,589,934,592 terabytes
  • 8,589,934,592 terabytes / 1,024 terabytes per petabyte = 8,388,608 petabytes
  • 8,388,608 petabytes / 1,024 petabytes per exabyte = 8,192 exabytes
  • 8,192 exabytes / 1,024 exabytes per zettabyte = 8 zettabytes

That’s a lot of data or, as my wife once said, a lot of dog food.

One thing to bear in mind though is that GPT by itself can only make hard drives over 2TB fully addressable if they are not the system drive (i.e. the drive from which the computer boots) so if you are using a 3TB drive as a data drive but use a 128GB SSD as your boot drive you’ll be fine. Once you boot into Windows or another operating system that supports GPT drives you’ll see the full 3TB on the data drive.

The other piece to the puzzle is EFI (Extensible Firmware Interface). EFI provides the required capability to boot from GPT drives and supersedes BIOS (Basic Input Output System) that has been around for decades . Unfortunately, many computers do not use EFI and continue to use BIOS except for Apple in its line of computers released over the last few years. Hopefully, this will start to change soon and new computers will have EFI as standard but does nothing for those of us with non-EFI systems which means those in that situation will have to use a drive under 2TB for boot purposes or shell out for new components or a whole new system.

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