New 21.5-inch Retina 4K models expected.
iFixit Tears Down New iMacs, Finds Soldered CPU on 21.5" Model
Both the 21.5-inch and 27-inch models include the new 802.11ac wireless standard, while the 27-inch model includes the new PCIe-based flash storage that was originally introduced in the MacBook Air back in June.
The 21.5-inch model has a few differences from its larger sibling, according to iFixit. It now includes a new empty PCIe SSD slot so users can upgrade the base model iMac to the Fusion Drive combination HDD/SDD drive that Apple introduced last year. In last year's 21.5-inch model, Apple did not include an empty PCIe slot on the logic board.
More disappointing, for users who like to upgrade their machines, is the fact that the CPU is soldered directly to the logic board, making an upgrade nearly impossible. iFixit gave the 21.5-inch model a 2 out of 10 for repairability, largely because of the soldered processor.
The 27-inch model received a 5 out of 10. The largest issue with upgrading the iMac is getting the glass and LCD panel out of the machine, making repairs impossible for all but the most determined do-it-yourselfers.