Command-Tab App Switcher doesn't work (related to corrupt .plist file error?)

Question

Hi folks,
The other day, Drive Genius 4 reported on a file corruption:  ~/Library/Preferences/System Mac.plist file. I tried to find the file but it didn't appear to exist.
Today, a strange phenomenon occurred which I think may be related. Strangest thing. I was using Safari, had several tabs open when, suddenly, I lost all input to any links in Safari and system wide as well, other than the cursor being able to move around the display. This dysfunction also included the Command-Tab App Switcher, though they keyboard text function still worked. I could enter text in fields.
I tried Force Quit Apps but I had to shutdown and reboot using the Power button. The reboot seemed to remedy the situation and most controls and keyboard functions returned except for the Command-Tab App Switcher.
In Safari, I used History/Restore Last Open Page, and when the searched for pages came up, they were blank - I was informed that there was no internet connection. Wi-Fi showed still on but grayed out, which could be logical. Also, this is when I noticed that the key repeat function wasn’t working. In System Prefs, I did manage to change the key repeat rate, which accelerates the backspace erase function but there is no key repeat, itself.
All of this, of course, might be pointing to that system file error. Whether it is that hidden Mac.plist file corruption I don’t know.
In any event, first thing I did was to run Permissions Repair. At one point, about 2/3rds of the way in, the progress bar hung for a few minutes and then finished the last third in a split second. In the Repair Permissions log there was no /Mac.plist file among those repaired.
Additionally in Sys Prefs, I tested for Tab key response by refocusing the radio buttons in the Full Keyboard Access option under Keyboard/Shortcuts panel from "Text boxes and lists only" to "All controls.” The refocus worked because now pressing Tab does move the cursor through every control. But still no Command-Tab App Switcher. In Spotlight, I did find the App Switcher.app but it would not open, because it is “Damaged or Missing.” App Switcher preferences are  also not accessible.
In Terminal I changed directories and do see the /Mac.plist file under discussion. I suppose I could -rm the file but I’m reluctant without knowing more about what other anomalies might result. The App Switcher is one of the Mac’s best utilities (thanks to er, Microsoft.) If the /Mac.plist file will refresh from the current status, no problem, I would think.
Has anyone experienced anything like this? Any suggestions? Thanks a whole lot in advance for your time and consideration in this!
All best,
David

Answer 1

First, get rid of the worthless, time-wasting "Drive Genius" according to its developer's instructions. Never install any so-called "utility" again.
Don't waste time repairing permissions again, either, unless you have an indication of a permission error involving system files, which you don't and probably never will.
Never delete any file in the shell (Terminal) for any reason.
There is no built-in application called "App Switcher." There is no built-in file named "System Mac.plist" either.
If command-tab application switching still doesn't work after you get rid of "Drive Genius," see below.
Please read this whole message before doing anything.
This procedure is a test, not a solution. Don’t be disappointed when you find that nothing has changed after you complete it.
Step 1
The purpose of this step is to determine whether the problem is localized to your user account.
Enable guest logins* and log in as Guest. Don't use the Safari-only “Guest User” login created by “Find My Mac.”
While logged in as Guest, you won’t have access to any of your documents or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this behavior; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.
Test while logged in as Guest. Same problem?
After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.
*Note: If you’ve activated “Find My Mac” or FileVault, then you can’t enable the Guest account. The “Guest User” login created by “Find My Mac” is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.
Step 2
The purpose of this step is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login, by a peripheral device, by a font conflict, or by corruption of the file system or of certain system caches.
Please take this step regardless of the results of Step 1.
Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards, if applicable. Start up in safe mode and log in to the account with the problem. You must hold down the shift key twice: once when you turn on the computer, and again when you log in.
Note: If FileVault is enabled in OS X 10.9 or earlier, or if a firmware password is set, or if the startup volume is a software RAID, you can’t do this. Ask for further instructions.
Safe mode is much slower to start up and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal startup may also be somewhat slow.
The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.
Test while in safe mode. Same problem?
After testing, restart as usual (not in safe mode) and verify that you still have the problem. Post the results of Steps 1 and 2.