Last week I tried to update my system from Lion to Mountain Lion but the installation failed and claimed the hard disk was damaged and couldn’t be repaired. After that my Macbook would only boot up the Mountain Lion installer continuously and wouldn’t let me access again my Lion disk.

I spent few nights trying hundred of workarounds and when I was about to erase the disk the light came. After trying to repair the volume with Disk Utility I saw the problem was an Invalid volume file count error.

That’s what I did to both up again my Lion installation, fix the problems in the hard disk and install Mountain Lion. You will need Disk Warrior and an external hard disk.

Recovering your previous Mac OS installation.

• Boot in safe mode. Restart your mac and after the chime press and hold the Shift key until you see a progress bar.
• If you haven’t done it yet, install Disk Warrior.
• Go to Preferences > Startup Disk and select the volume that contains your old operating system.

Creating an alternative boot up volume

• Clone your hard disk using Carbon Copy Cloner, OR
• Create an eDrive startup disk with Tech Tool.

Fixing the disk

• Boot from the Disk Warrior CD -> It didn’t show the volume with my old Mac OS system.
• Boot from the recovery partition and try to fix the problem with the Disk Utility -> Disk Utility wasn’t able to fix the problem.
• Boot in single user mode and try to fix the problem with fsck and fsck_hfs -> fsck wasn’t able to fix the problem.
• Check this thread from the Apple Discussions, where Karl other useful workarounds.