http://www.macnn.com/articles/13/01/07/upgrade.now.available.on.all.imac.models/
Fusion Drive becomes option on low-end 21.5-inch iMac
updated 09:34 am EST, Mon January 7, 2013
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Upgrade now available on all iMac models
Apple has expanded options at its online store, allowing the Fusion Drive on customized orders of the low-end 21.5-inch iMac. Previously, only 27-inch and higher-end 21.5-inch models could get the upgrade. Picking a Fusion Drive adds $250 to the low-end 21.5-inch's pricetag, and still yields a total 1TB in storage.
Fusion Drive is Apple's name for a hybrid configuration in which the OS and other commonly-used files are stored on an SSD, while everything else is kept on a conventional hard disk. Both drives are treated as a single unit, but with the added benefit of faster loading times for the OS and certain apps.
