Apple is now offering the Fusion Drive as a build-to-order option when ordering the most affordable 2.7GHz 21.5-inch iMac model, a change in availability from October when the system was limited to high-end versions and the 27-inch iMac.

The revision to the Online Apple Store's build-to-order options, first spotted by French blog MacGeneration and confirmed by AppleInsider, adds Apple's hybrid Fusion Drive as a $250 upgrade to the base model 21.5-inch iMac.

Introduced in October 2011 alongside the redesigned iMac, Apple's Fusion Drive is a hardware and software solution that promises the performance of a solid state drive with the storage capacity of a hard drive. Initial tests showed the hybrid drive to cut startup times in half while increasing read and write speeds significantly when compared to a traditional 5400-rpm hard drive.

The software driving the technology is built into OS X Mountain Lion and join a 128-gigabyte SSD with either a 1-terabyte or 3-terabyte HDD to form a single addressable volume. Integral applications and the operating system itself are permanently stored on the flash memory while other files are store on the slower spinning drive. The system monitors a user's operations and swaps frequently accessed files, folders or programs between the two to optimize performance. Transfers take place seamlessly in the background and require no user input.