APPLE INSIDER
Apple has updated the storage options across the board of its 21.5-inch iMac range by switching to SSD storage as the default, with users now only able to swap out the SSD with a Fusion Drive in their purchase.
As part of Tuesday’s refresh of the iMac lineup, the 21.5-inch models now have upgraded base storage options. All models now have a 256GB SSD as standard, a first in the model’s history.A solid state drive (SSD) offers some benefits over a hard drive, with the chief one being generally faster reading and writing times. As an SSD doesn’t rely on spinning discs like mechanical hard drives do, they are also more resilient to being moved around while power is applied, making them extremely useful for portable devices like a MacBook Pro.
Source: APPLE INSIDER