Download of iMac OS X 10.8.5 Supplemental Update 1.0 for Mac was on the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The program lies within System Tools, more precisely General. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe.
From the developer:
The iMac OS X 10.8.5 Supplemental Update 1.0 is recommended for iMacs (Late 2013) using NVIDIA GeForce GT 750M graphics and running OS X 10.8.5 Mountain Lion.
This update: Fixes an issue that may cause external drives to be ejected after the computer goes to sleep Fixes an issue that may prevent certain USB Bluetooth adapters from working
You may want to check out more software for Mac, such as OS X Bash Update or Learn Mac OS X, which might be related to iMac OS X 10.8.5 Supplemental Update.
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