You can find many IHV (independent hardware) companies like LaCie offering RAID subsystems, in which many cases they utilize 4 units of 500GB hard drives.
For the ease of installation, products aimed at SOHO and individual enthusiasts are often built in tower chasis.
For corporates, a 2TB unit is made out of 4 x 500GB drives more often laid out in a "rack" (imagine something like a flat big tray.)
There are solutions like stacking multiple racks to be connected through fiber channels to the server. If you had 7 racks stacked that'll make 14TB, for example.
In reality, however, corporate storage solutions would generally sacrifice a lot of raw capacity for security and backup. They use techniques like RAID to reserve a part of capacity to achieve higher reliability. In other words, the capacity made available to store data is much less than plain sum of the disk capacity used.
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