Screen on iMac keeps refreshing can not do anything. I think it's the finder.

Try starting in Safe Mode and see if the problem still occurs?

Restart holding the "shift" key.

(Expect it to take longer to start this way because it runs a directory check first.)

If that works then go to your Finder "Go" menu hold the option key to choose "Library". Then go to Preferences/com.apple.Finder.plist and delete it. Restart normally and test.

It may also be from a third party Login Item, so if deleting the pref doesn't solve it

look in System Preferences > Users & Groups > Login items and delete any third party login items (-), you can always add them back with the (+). Then restart normally.

You can reboot Finder (Option+Command+Escape) but to replace Finder you have to reinstall OS X.

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Launch the Console 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 Console in the icon grid.

Step 1

Make sure the title of the Console window is All Messages. If it isn't, select All Messages from the SYSTEM LOG QUERIES menu on the left. If you don't see that menu, select

View ▹ Show Log List

from the menu bar.

Enter the name of the crashed application or process in the Filter text field. Select the messages from the time of the last crash, if any. Copy them to the Clipboard (command-C). Paste into a reply to this message (command-V).

When posting a log extract, be selective. In most cases, a few dozen lines are more than enough.

Please do not indiscriminately dump thousands of lines from the log into this discussion.

Important: Some private information, such as your name, may appear in the log. Anonymize before posting.

Step 2

In the Console window, look under User Diagnostic Reports for crash reports related to the crashed process. The report name starts with the name of the process, and ends with ".crash". Select the most recent report and post the entire contents — again, the text, not a screenshot. In the interest of privacy, I suggest that, before posting, you edit out the “Anonymous UUID,” a long string of letters, numbers, and dashes in the header of the report, if it’s present (it may not be.) Please don’t post shutdownStall, spin, or hang logs — they're very long and not helpful.