"screen shot can't be saved" and other permissions issues

Question

After having my iMac at the Apple Store for four days solving a problem with "quit unexpectedly" issues with all Apple built-in Apps on OS X Yosemite, I got the machine back with those problems solved.  It involved re-installing OS X and restore files from backup.  I thought all was well; the Genius Bar dude showed me that it's fixed. He said it took so long because of bizarre permissions issues he'd never seen before.
Now that I've lugged this 40lb machine back home (after verifying the fixes in the store) I now find that several (other) functions don't work.  When I try to do a screenshot, I get this message:"Your screen shot can't be saved.  You don't have permission to save this file in the location where screen shots are stored."  Message could have been more helpful; the "location" is the desktop.  Pretty descriptive, though.  Seems I can't save anything to the desktop.
I tried creatng a folder on the desktop.  Got this message: "Finder wants to make changes. Type your password to allow this".  Type password and new folder appears.  Drag the new folder to the trash, get same message..
I tried copying a file from a network drive to the desktop. I get this dialog: "Modifying Desktop requires and administrator name and password.  I clicked "authenticate" and after a 3 minute delay (with "Preparing.." ) I get this dialog; "Finder wants to make changes.  Type your password to allow this".  I type the password and the file appeared on the desktop.  Before the work at the Apple Store, this never happened.  BTW I have only one user account and it's marked "Allow user to administer this computer".  Thinking something might be goofy with the pw, I changed it (took 3 minutes to do, oddly) and rebooted. No joy. Same thing happens when I try to drag something from the desktop to the trash.  Long time "moving" message followed by having to provide and admin pw.
I tried to save a Safari attachment to the Downloads folder and got this message: "Safari could not download the file xxxxx because there is not enough free disk space".  Since I have more than 600GB of available space, I think the message is wrong and that it's really a permission issue with the Downloads folder.
Tried to reset the password again.  Click to unlock the Users & Groups panel, get "System Preferences is trying to unlock Users & Groups preferences.  Type your password to allow this".
All this tells me that somehow Yosemite is in "nag" mode (reminiscent of Windows Vista.
Any ideas?
Chaz

Answer 1

Back up all data before proceeding.
This procedure will unlock all your user files (not system files) and reset their ownership, permissions, and access controls to the default. If you've intentionally set special values for those attributes on any of your files, they will be reverted. In that case, either stop here, or be prepared to recreate the settings if necessary. Do so only after verifying that those settings didn't cause the problem. If none of this is meaningful to you, you don't need to worry about it, but you do need to follow the instructions below.
Step 1
If you have more than one user, and the one in question is not an administrator, then go to Step 2.
Triple-click anywhere in the following line on this page to select it:
sudo find ~ $TMPDIR.. -exec chflags -h nouchg,nouappnd,noschg,nosappnd {} + -exec chown -h $UID {} + -exec chmod +rw {} + -exec chmod -h -N {} + -type d -exec chmod -h +x {} + 2>&-
Copy the selected text to the Clipboard by pressing the key combination command-C.
Launch the built-in Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.
Paste into the Terminal window by pressing command-V. I've tested these instructions only with the Safari web browser. If you use another browser, you may have to press the return key after pasting.
You'll be prompted for your login password, which won't be displayed when you type it. Type carefully and then press return. You may get a one-time warning to be careful. If you don’t have a login password, you’ll need to set one before you can run the command. If you see a message that your username "is not in the sudoers file," then you're not logged in as an administrator.
The command may take several minutes to run, depending on how many files you have. Wait for a new line ending in a dollar sign ($) to appear, then quit Terminal.
Step 2 (optional)
Take this step only if you have trouble with Step 1, if you prefer not to take it, or if it doesn't solve the problem.
Start up in Recovery mode. When the OS X Utilities screen appears, select
          Utilities ▹ Terminal
from the menu bar. A Terminal window will open. In that window, type this:
res
Press the tab key. The partial command you typed will automatically be completed to this:
resetpassword
Press return. A Reset Password window will open. You’re not going to reset a password.
Select your startup volume ("Macintosh HD," unless you gave it a different name) if not already selected.
Select your username from the menu labeled Select the user account if not already selected.
Under Reset Home Directory Permissions and ACLs, click the Reset button.
Select
           ▹ Restart
from the menu bar.