Ok, I know, I was a fool. I had the brilliant idea to mess up the permission of my whole disk. I just wondered why I was not allowed to drop a file at the root of my HD. So I right-clicked on the HD icon, run "Get Info", and at the bottom I saw that the permission for "everyone" was "read only". This looked wrong to me, it is my hard disk, I am the only user of this computer, so it looked perfectly sensible to me to set the permission to "read and write" for everyone. But, not happy of doing just that, I decided to apply this setting also to all the files and folder of my HD.... Result: the system is now completely screwed up, and cannot boot anymore, even in Verbose or Single User mode. In verbose mode, I see some lines complaining that several PLIST files have "wrong permissions", so they are not processed... Is there any way to reset the permissions of the whole filesystem (or at least of the critical system files) to their original values, so that the system can, at least, boot up form disk? I am now booting from an external USB disk, where luckily I did store (a couple of months ago) a copy of my main HD with Carbon Copy Cloner... Or do I have to re-install OSX LION from scratch?