Can't mount dmg. But works under new user

First, back up all data.

Hold down the option key and select

Go ▹ Library

from the Finder menu bar. From the Library folder, move the entire contents of the subfolder "Caches" (not the folder itself) to the Trash, then log out and log back in. Empty the Trash. Test.

If the problem is not solved, open the Library folder again in the same way and move the entire contents of the subfolder "Preferences" (not the folder itself) to a new folder on the Desktop (not to the Trash.) Log out and log back in. The account will then be pretty much in a clean state. Test. If you can still reproduce the problem, then put the Preferences folder contents back, replacing any that were created in their place, log out, and log in again.

If you can't reproduce the problem in the clean state, then start putting things back in the Preferences folder piecemeal, starting with items that seem unrelated to the issue. I can’t be more specific. After each group of items, repeat the log out/log in cycle and test. At some point, the problem may reappear, in which case you know that something in the last batch of files you restored is at fault. If it never reappears, the problem is solved.

This may be a very time-consuming process, but it’s the best I can suggest in the circumstances.

I can't make sense of the facts you've stated in this thread. I've never seen a story like yours, and I can come up with no explanation for it. I suspect a misunderstanding, but I don't know what it is.

The last resort, when a problem is truly inexplicable, is to erase the boot volume, reinstall OS X, and restore from a backup. Once in a while, this action will cause a bad sector on the boot drive to be mapped out and produce miraculous results. But only once in a while.

If you prefer to create a new account, don't restore your user data in Setup Assistant. Transfer your files manually from a backup.