A 2TB drive could present as many as (2 * 1024 * 1024 * 1024 * 1024 / 512) logical 512-byte sectors to the host. If you're keeping track, that's 2^32, or the maximum that can be represented by a 32-bit sector number.

The standard MS-DOS partition table layout, used by most PCs, has a 32-bit field for starting LBA (sector number). Similarly, a second 32-bit field is used for the sector-count for a partition.

Any drive with more than 2^32 sectors (2TB) might be unable to boot from sectors beyond the first 2TB, because the standard MS-DOS partition table has no way to encode higher capacity values in a way that the BIOS understands.

There are lots of ways to work around that. Eg. boot from a partition within the first 2TB, and then let the operating system use a different scheme to identify where the rest of the disk partitions are located.

But disk drive makers, like most computer hardware companies, are reluctant to build things that require any kind of special handling for the typical Windows machine.

So, we've been "stuck" at 2TB for (only) the past year because of that limitation.

Cheers