New 21.5-inch Retina 4K models expected.
Maximum MacBook Air Drive 80GB for Now
In its pursuit of thinness, the MacBook Air uses a 1.8" single-platter 80GB HDD which measures only 5mm high. While larger capacity 1.8" drives exist, they use double-platter designs which result in an 8mm height. This extra 3mm explains why the 80GB drive is the only HDD option available for the MacBook Air.
Technology, of course, marches forward, but the last we've heard about higher capacity single-platter HDDs came from Toshiba in September 2007. At the time, Toshiba had prototyped a 1.8" HDD that fits 120GB on a single platter, but this drive is not expected to come to market until 2009.
Those who can afford the Solid State Drive (SSD) may be in in some luck, as Samsung did announce (in Jan 2008) that a 128GB SSD drive is coming in the "first half of 2008". This 1.8" drive multi-level cell flash drive will be produced in the 1.8" 5mm high size, although the announced interface (SATA) is not the same as that used in the MacBook Air (PATA). Pricing has not yet been announced on the 128GB SSD drive, but will certainly carry a significant premium.