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Computerworld - Apple on Monday updated Snow Leopard for the second time, fixing a data deletion flaw and an iMac performance issue, while blocking Atom-based netbooks from running its operating system.

Some people, however, disputed Apple's claim that it had fixed a major performance problem on their new iMacs, saying that the update to Mac OS X 10.6.2 had not resolved the extremely sluggish playback of Flash-based video. "It appears that my problem largely remains," said a user identified as "newfruits" on a support thread dedicated to the iMac performance issue. "No improvements. Overall CPU usage still exceeds 110%."

Two weeks ago, users reported that their new 21.5-in. and 27-in. iMacs -- models Apple unveiled on Oct. 20 -- were having trouble playing Flash-based video on sites such as YouTube. "All the videos play really jerky, with sound popping ... so I opened Activity Monitor and Flash Player (only thing not running 64-bit) is spiking to 105% CPU usage every few seconds," one user said on Apple's support forum at the time.

In the notes accompanying yesterday's update, Apple hinted that its fix might not solve everyone's video problem. "This update addresses video playback and performance issues for iMac (21.5-inch, Late 2009) and iMac (27-inch, Late 2009) computers that may occur in some situations while AirPort is turned on," Apple stated.

The update boosted the speed of some iMacs. "I had the same problem on a 27-in iMac and now, after the update, everything is OK," said "nunoalmart."

Others, however, saw no improvement. "I've just installed 10.6.2 and it doesn't make any difference to the Flash video playback problem," countered "Grant P" on the same thread. "Still get same >100% on the CPU."

On the support forum, users have forwarded theories to explain the problem, ranging from a hardware defect or a problem with the operating system to a bug in Adobe's Flash Player.

Mac OS X 10.6.2 also addressed a Snow Leopard bug that deleted data when users logged into a "guest" account after upgrading from Leopard. "[A fix was provided for] an issue that caused data to be deleted when using a guest account," Apple said in the update's advisory.

Apple apparently stripped support for Intel's Atom processor from Snow Leopard with version 10.6.2, according to reports on several netbook-oriented message forums and blogs. The Atom is popular on the lowest-priced netbooks, which typically run Windows XP or Windows 7, but some owners have created a "hackintosh" by installing Apple's OS on their small-sized systems.

"I can tell you 10.6.2 DOES NOT WORK with the [Dell] Mini10v," said a user labeled as "Crisu" on a message forum hosted by MyDellMini.com. "OS X tries to start, but it hangs before you can see the Apple and system is trying to boot again ... you are in a loop."

Rumors had circulated two weeks ago that Apple was planning to block Atom-based netbooks from running Snow Leopard.

Users said that a workaround that involves copying the kernel from Mac OS X 10.6.1 to replace the revised kernel packaged with 10.6.2 lets them run the update on their netbooks.

Apple also patched a problem with the four-finger swipe gesture on MacBook and MacBook Pro notebooks in 10.6.2, and included other fixes for the Mail application, the Safari browser, the MobileMe sync and storage service, fonts and network drives.

Mac OS X 10.6.2 can be downloaded from the Apple site, or installed using the operating system's integrated update service. Included in the update are patches for 58 separate security vulnerabilities.

Read more about Mac OS in Computerworld's Mac OS Topic Center.