As Apple rolled out other models of Mac laptop and desktop systems, SSD's became a Build-To-Order option in select models, often at a stiff premium. Many Do-It-Yourselfer's opted to perform an SSD upgrade themselves after the purchase. With the majority of SSD's in the 2.5" laptop drive format - most were drop-in replacements in MacBook and Mac mini systems. A fewfull-size 3.5" Solid-State drives arrived, many used disk adapter brackets and cases in Macintosh Pro towers and iMac models.
In the years since - SSD storage has made mind-blowing performance improvements. Drive capacity increased at ever more affordable prices and performance doubled - then quadrupled as rapidly evolving SSD controller chipsets and Firmware advanced.
It's the increased density of the flash NAND chips that's pointing towards the future. The latest MacBook Air models with custom SSD module are pointing to a future where our notion of a 'hard drive' doesn't necessarily have to occupy the paperback size space of a 3.5" drive, or a laptop disk's deck-of-cards form factor. To deliver ever more compact and lightweight mobile computers - our current notions of traditional spinning platter drives and their physical bulk is likely to end sooner than you think.
As with the new MacBook Air's, an off-the-shelf retail SSD drive upgrade or replacement limits options as Apple implements a proprietary mini SSD card in a different form-factor. OWC is the sole manufacturer who offers higher-capacity (and somewhat more expensive) MacBook Air SSD card upgrades as an alternative for these new Mac laptop models.
Flash memory speeds used in Solid-State drives are improving. They recently began to push beyond the bandwidth capabilities of the SATA II interface, and further speed gains are available to those with SATA III enabled computers. You can safeguard your SSD purchase by looking to the Micron/Crucial 'C' or 'M' Series, intel 3xx-5xx, or OCZ '3 & 4' line of drives that sport a SATA III interface and are fully backwards compatible with SATA I & II in the meantime.
Lexar Media - Crucial 128 GB M4 SATA III SSD
Intel 510 Series 120 GB SATA III SSD
OCZ 120 GB Vertex 4 SATA III SSD
The emergence of 5Gbps USB 3.0 and Intel and Apple's 10Gbps ThunderBolt ports provide interesting opportunities for ultra high-speed backup and data transfers using external Solid-State drives. Given the longevity of solid-state drives and the long lifespan free from the mechanical failures of conventional hard drives, your SSD might serve you on your current system - and in a USB 3 drive enclosure or ThunderBolt drive case further down the road.
FYI: It's critical to make sure Seagate's Hybrid has the latest firmware applied. A bootable ISO disc updater is available at Seagate's site for Mac OSX users.
These laptop size SATA III Hybrid drives offer from 250GB up to 500GB capacity 7200 RPM spinning platters - plus SSD flash memory built into the drive. Onboard firmware manages and stores your most frequently used data on the SSD portion for higher performance. And because this data optimization and caching is handled on the drive itself -- these Hybrid SSD drives are cross-platform compatible and work well on Windows or Mac OSX without requiring any special drivers or software. With prices ranging from $100-150 you can get extra capacity you may need, while still enjoying _some_ of the performance benefits of Solid-State drives on your Mac.
Note Cruicial's SSD was tested on a Windows PC with a SATA III bus for spectacular performance, OCZ's Revo is a PCI-e Slot RAID SSD that few mortals can afford - and has yet to be qualified for Macintosh use. Corsair's adoption of SandForce and Indilinx controllers in their drives puts their best SSD's on a par with the Vertex 2.