Apple Updates iMac FirmwareApple Updates iMac Firmware

Apple has released a firmware update for its 27-inch iMac, fixing problems with the all-in-one computer's graphics cards.

The fix is for the ATI Radeon HD 4670 and 4850 graphics cards made by Advaced Micro Devices. The update corrects unspecified problems that Apple says "may cause image corruption or display flickering."

Buyers of the 27-inch iMac have posted numerous complaints on the company's support forum. Among the problems cited were flickering, cracked, or yellow-tinged screens. Apple declined to comment on the problems.

The 27-inch iMac, the largest of the line, was introduced in October, along with a 21.5-inch model.

The computers have been very popular, at least among early buyers who are typically Apple buffs. Mac sales at U.S. retailers in October and November were up 74% from the same period a year ago, with the majority of those sales iMacs, according to NPD Group.

The large number of orders could explain a delay in shipping 27-inch iMacs, experts say. While the 21.5-inch version ships in 24 hours, the larger model takes two weeks. Apple has apologized for the delay, saying it is "working hard" to fill orders.

Despite the strong early sales, its too soon to say how successful the latest iMacs will be. Analysts will be watching whether the trend continues beyond early adopters. Sales of the 27-inch model will be particularly interesting because research shows that people typically prefer desktop monitors at around 21 inches.

Along with Mac desktop sales, Windows desktop sales also have been on the rise. During the same October/November time frame, sales of Windows desktops rose 5% from a year ago, according to NPD. The increase is seen as significant, because sales have been down each of the previous months in 2009.