updated 12:01 am EDT, Fri October 25, 2013
Only for 'late 2013' iMacs, fixes Power Nap and USB issues
On Thursday, Apple launched a firmware and software update for the most recent models of iMac, known as the "late 2013" or "Haswell" versions. The first is an SMC update that resolves an issue with Power Nap (a feature that is new to iMacs, but has been included with select MacBook Pro models for a while), and an OS X 10.8.5 "Supplemental Update" that further resolves miscellaneous USB issues only for late 2013 iMacs that came with an Nvidia GeForce GT 750M.
The firmware update is called "iMac SMC Firmware Update 1.1," and fixes an issue with Power Nap that could cause the fans to spin up even when they weren't actually required. The idea behind Power Nap is that it allows the computer to execute certain kinds of maintenance and updating while sleeping, and the update allows it to do so silently as intended.
The Mac OS X 10.8.5 Supplemental Update is, as the name implies, only for those late 2013 iMac users running 10.8.5. Additionally, iMacs needing this particular fix must have an Nvidia GeForce GT 750M video card, suggesting the problem originates with the card. The update fixes various USB issues, including a problem that prevented certain USB Bluetooth adapters from working, and more importantly a bug that randomly ejected USB storage devices when going to sleep.
Owners of the affected model can obtain the updates through Software Update. They can also be downloaded directly from Apple's Support Downloads website.