Apple has pushed out two separate updates for the late 2013 iMac. The first update, labeled “iMac SMC Firmware Update 1.1,” addresses an issue with Power Nap on flash-based iMacs that could cause the fans to begin spinning even when they weren’t needed. Instead the updates will now occur silently.
The second of the two updates is a Mac OS 10.8.5 supplemental update for late-2013 iMacs with NVIDIA GeForce GT 750M. The update fixes multiple issues with USB devices, such as the inablitly to use some USB Bluetooth adapters, or the Mac randomly ejecting some external storage devices while going to sleep.
Both updates can be downloaded through Software Update in the Mac App Store, or you can download them individually from Apple’s support page.