Fusion Drive specifications

1TB Fusion Drive consists of a 7200 RPM HDD and a 24GB SSD. macOS is installed on the SSD. You might prefer getting the 2TB Fusion Drive with a 128, 256, 512, or 1TB SSD option.

Frankly, given the way the tech market is moving, I would recommend just getting an SSD instead of a Fusion Drive. If you need larger storage capacity, then add external HDD storage. The SSD should be at least 512GB. RAM should be at least 16GBs unless it is user upgradeable in which case get the bottom amount and add your own third-party RAM which is cheaper than what Apple charges.

Built to order purchases must go through the online Apple Store. Local Apple Stores only carry models with default options.

FWIW, for 2019, the SSD's are now 32GBs for all but the 27" 2TB Fusion drive which come with a 128GB SSD. The 21.5" Fusion iMacs come with a 5400 rpm HDD, the 27" Fusion iMacs come with a 7200 RPM HDD.

There is no 2TB Fusion Drive with a 256, 512, or 1TB SSD option.