On the most expensive 27-inch iMac, the upgrade cost to a "256GB Solid State Drive" is $400. But the upgrade cost to a "1TB Serial ATA Drive + 256GB Solid State Drive" is only $100 more - $600. Am I missing something here? Is a "Serial ATA Drive" a worse kind of SSD than the 256GB one? I'm just perplexed at how cheap the price is compared to the SSD upgrades for their other computers.