I've seen that type of problem before . Unfortunately not sure what causes it. 2nd life runs find on my system. Have you tried setting up a test user account: Setting up a Test Account in OSX (click on link).

Also like happened to me recently, open disk utility from the Utilities folder, see if that particular disk image is shown. If yes, choose the lower image & choose eject. Then choose the main disk image & choose mount. That should fix your problems with luck.