Hard drive technology is continually evolving. Larger capacity hard drives that were never dreamed of ten years ago are on the horizon. The actual physical size of hard drives also continues to decrease. The older hard drives contained 5 1/4" platters. The newer drives contain 3 1/2" or smaller platters.
Newer removable hard drives allow you to physically remove the hard drive at the push of a button. PC users who require large amounts of storage use these drives. They are also great to secure your data. After turning off your PC, pop the hard drive out, and store it in a secure location.
Removable hard drives also continue to get physically smaller, while increasing in storage capacity. Laptop PCs also use a smaller version of these drives.
In short, the sky is the limit! Hard drives will continue to shrink physically, while providing larger storage capacities.