Last week, Seagate announced the first 3TB single external hard drive (shown at left). This seemed a little early, given that Hitachi announced the first 1TB drive in 2007 and  Western Digital announced the first 2TB last spring. So I was interested in the technology behind the drive. Usually, new drive capacity comes out first as internal drives and only later in the form of external drives. But the FreeAgent Desk--an external drive--is the first 3TB drive. And there's a good reason for that.

Your operating system communicates with a drive through the systems' BIOS and a partition table in the first sector, typically known as the master boot record (MBR), which keeps track of the starting points of each sector on the disk. Under a scheme that the hard-drive industry agreed to in 1980, the maximum storage this can address would be 2.1TB. This limitation is part of nearly all BIOSes and some operating systems even today, according to Seagate's director of product management, David McKenzie.

In the traditional scheme, the BIOS and MBR expect drives to be laid out with a sector header, followed by 512 bytes of data, followed by the next sector header. Seagate and the rest of the industry are now moving to 4K sectors, known Advanced Format, with most drives slated to be shipped after January having this format. This would theoretically allow up to 8 times more storage (17.6TB) on a drive, but typically today's BIOSes don't support this.

Instead, Seagate needs hardware and firmware that makes the drive appear as though it uses the 512 byte scheme but actually converts to the 4K scheme when reading the drive, giving it compatibility. That's possible because external drives use drivers to communicate, something that can't be done with the boot drive. (This isn't the first external drive to do this.) But as a result, we're seeing this capacity first in external drives, rather than internal ones, as the industry wants its internal drives to work to full capacity in all systems.

Over time, you can expect updated BIOSes that support 4K sectors, though we still need operating systems to properly support this. Windows Vista, Windows 7, Mac OS X, and some versions of Linux do, although there are still issues with some of the drive utilities. In fact, newer operating systems, such as Windows Vista and Windows 7, support a replacement for the MBR called the GUI partition table (GPT), which supports a much larger address space--around 8 zettabytes, a truly enormous number.

Often, when I think about higher capacity drives, I think about higher areal densities, meaning platters that store more data on them. That's not particularly the case here: The actual density hasn't really increased significantly over Seagate's early 2TB external drive. Instead, the increase in capacity comes from two primary changes. 

First, the drive consists of five platters rather than four, through new technology that lets the company put the platters closer together. Second, moving to 4K sectors increases capacity over 512 byte sectors. McKenzie, told me that areal density will continue to improve over the years, but it's just not the driving factor behind this particular drive.

It's pretty amazing to step back and think about how quickly capacity has grown. My first hard drive had 5MB of storage, about the size of a single MP3 song or maybe a single JPEG image. When I started at PC Magazine in 1991, the largest hard drive available was 130MB, which now would hold about 2 seconds of HD video taken with a Flip HD or other camera. Today, we're talking about 3 terabytes, which Seagate says is enough for 120 HD movies. That's a huge increase.

To fill such a drive, you need to have a lot of video content, or a whole lot of very specialized data, at least for an individual PC. That's more storage than I need right now--but we all know that storage seems to expand to fit the existing space.

Originally posted to Michael Miller's blog, Forward Thinking.