Right off the bat I'll admit it was stupid to jump into trying to install a new version of OS X to the only partition without having taken adequate precautions (no backups or recovery disks), but unfortunately it's done. (the failed installation attempt, not the installation itself!)
I'm on a mid-2011 MBP that was running Lion. I download the installer for the latest OS X beta version to my HD and started installation process from there. The system rebooted and the installation started, but after 1 minute into the estimated 43 minutes for the installation to complete, I get an "Install Failed" message: "OS X could not be installed on your computer. The OS X upgrade couldn't be started because the disk Lion DP GM is damaged and can't be repaired. After your computer restarts, back up your data, erase your disk, and try installing again. Click Restart to restart your computer and try installing again."
Restarting just repeats the process and the same problem recurs. I went into "Disk Utility" from the Installer's menu, and trying to verify the offending volume returns "This disk needs to be repaired. Click Repair Disk". Unfortunately the "Repair Disk" option stays greys out.
Is there any way I can "revoke" the installation process (via the Terminal perhaps?) and get back to my previous set up with Lion (without losing data, obviously), so I can start over the right way this time?
Any advice is appreciated.
Update:
I'm not able to back up my main volume ("Lion DP GM") with Disk Utility. It barely starts before I get the following error:
Unable to create "Lion DP GM.dmg" (Input/output error)
I tried booting in recovery mode and reinstalling Lion, but it didn't work. Under the disk "Lion DP GM" where I would install it, it says
This disk is locked
When the OS installer is running, I can see – via Terminal – the contents of my main volume under /Volumes/Image Volume, whereas in Recovery Mode it's under /Volumes/Lion DP GM
Unless someone has some bright ideas how to restore things back to how they were, I would at least like to attempt backing up the entire "Lion DP GM" disk to an external disk via Terminal (since Disk Utility failed). What command would be the best way to attempt this?
Update #2:
In the end I copied just the bare essentials to an external drive via Terminal, which didn't cause any problems. Then I erased the whole partition, and did a USB installation, which completed successfully. Disk Utility on the new version of Mac OS X is no longer complaining of HD errors (although I'm not sure whether that means I'm in the clear or not).
Best Answer
If you haven't already made backups, you can start up from the recovery partition and save a disk image of the main OS X volume (or other volumes you use) to a different drive in Disk Utility.
The first thing I'd try would be to do an upgrade install of 10.7 by choosing Reinstall OS X without erasing anything first. If that wouldn't work, I'd do a clean reinstall and restore files manually.
See these Apple KB articles: