I have a 2010 Macbook Air with a hardware (temp sensor) problem. In order to make it usable, I deleted a plist file using the following process (found on the web): 1) Open System Information (under Apple icon in upper-left, click About This Mac, click More Info) 2) Click System Report button, under Hardware, find your "Model Identifier" -remember this for later use 3) Navigate to /System/Library/Extensions an right-click, Show Package Contents of IOPlatformPluginFamily.kext 4) Open Contents/PlugIns and Show Package Contents of ACPI_SMC_PlatformPlugin.kext 5) Open Contents/Resources 6) Find your MacBookAir?_?.plist (? = your model identifier from step 2) and delete the file 7) Reboot! Remember, anytime you do a System Update, you may need to repeat this process" Since doing that it has been running ok (first on Mavericks, then Yosemite). However, when I installed the El Cap public beta I found I could not delete the file, because it is 'locked'. I have tried to unlock it via 'Get Info' but the relevant tick boxes are greyed out and I cannot change the permissions. Option clicking and dragging to the trash doesn't work, nor does using the Terminal commands I read about online. Basically the system won't let me do anything to the file! I've re-installed Yosemite and have no problems deleting the file. To illustrate the difference doing this makes, the MBA has a single core Geekbench 3 score of around 1000 with Yosemite (with the plist file deleted) and around 200 with El Cap (with the plist in place). Any ideas?