Heads and media are like silicon chips in that no two units are "exactly" the same. Yield in terms of usable capacity varies from one unit to the next just like

Heads and media are like silicon chips in that no two units are "exactly" the same. Yield in terms of usable capacity varies from one unit to the next just like clock rate for CPUs. When you buy an HDD it has been validated to operate correctly at the rated capacity, there's no guarantee how it may handle higher densities. Besides, unless you work at an HDD factory you have no way of knowing just how similar the parts of a particular 250GB drive are to a 1TB drive. For all you know they may have gotten a particularly dodgy lot of heads or patters which definitely can't handle 1TB.