Updating to a major new version of OS X can seem akin to walking through a mine field, albeit one with relatively few mines. In most instances, you'll be fine. But you never know when disaster may strike.
In my case, the OS X Yosemite upgrade went as smooth as silk for my 2012 MacBook Pro. I've been happily running it via beta versions for months. I waited until the release version of Yosemite before upgrading my 2009 Mac Pro. I expected things to go just as smoothly. Instead, I hit a mine.
The installation itself was a success. However, as soon as I attempted to use the Mac, all hell broke loose. The most serious symptom was that almost every action now proceeded at a snail's pace. Several apps--notably Safari, Tweetbot, Outlook, and even the Finder--often became entirely unresponsive. I had to repeatedly Force Quit these apps just to maintain a minimum level of response.
Other symptoms included an inability to use OS X's screen capture, a failure for custom desktop pictures and screensavers to be saved across a restart, and a loss of all audio. When I went to System Preferences to troubleshoot these symptoms, I quickly wound up with an endless spinning beachball. If I clicked the Sound preferences icon, the app immediately crashed.
What was going on? What could be causing such an odd multitude of serious symptoms? Was it the OS X software itself? Or a conflict with some existing third-party software? I had no idea. But I was determined to find out.
Spoiler: For those in a hurry to know the answer, I'll cut to the chase. The perpetrator of all the symptoms was a single obscure file (technically, a plug-in folder containing several files): InstantOn.driver. It's located in Volume/Library/Audio/Plug-Ins/HAL and is optionally installed as part of Rogue Amoeba's Audio Hijack Pro.
As it turns out, Rogue Amoeba has been aware of the incompatibility for some time. They posted a warning against using the Instant On driver soon after the developer version of Yosemite was released. Over the summer, they released Audio Hijack Pro (and Airfoil) updates that included a new compatible version of Instant On.
I never installed the update because I was unaware that it existed. I was still using a version from 2013 (and only had the app on my Mac Pro, which is why I didn't have the same problem with my MacBook). My lack of awareness was mainly because I hardly ever use Audio Hijack. The last time I even launched the app was sometime in the first quarter of 2014, so I never received any in-app notifications. Unfortunately, the Instant On plug-in remains active--and a potential source of trouble--even if you do not open its parent program.
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