Volume contains an OS X installation which may be damaged

"Leroy, Leroy, lama sabachthani"… Hope I am not a pain but am really stumped. To recap:

Got the following message “Volume contains an OS X installation which may be damaged. Quit the installer to restart the computer and try again”. I tried this without results. iMac mid-2008, 4GB Ram, with Mountain Lion 8.5 on internal HD and OS X 10.11 on external drive from which I could start with no problems. Now when OS X 10.11 is selected as start up system, computer starts normally, progress bar reaches approx. 30% and then screen turns black and computer shuts off. Tried repairs to no avail. Re-installing OS X from internet took 25+ hours but produced same message… When booting from internal drive with MAC OS X 8.5. start up is normal and external drive where OS X 10.11 is located, mounts on desktop with all data visible and accessible. Don’t want to initialize my external drive as this is a 6TB disk and I cannot back it up and restore. What to do?

Thanks Macjack,

I've done this repeatedly. Below is a copy of Disk Utilities log.

Problem is that I cannot make a backup of the affected disk which is a Lacie 6T Quadra drive. I have no other disk of that capacity which could hold such BUP so I cannot erase it and restore it. Tried to reinstall OS 10.11 but get the above mentioned message. Tried everything: Disk Utility, DiskWarrior, Techtools, Drive Genius, no joy. If push come to shove, I'll leave things as they are since I can still access my data when the Lacie drive mounts as external drive on the desktop when I start from the iMac with Mountain Lion 10.8.5, until such time I can borrow a 6 or 8TB drive to backup, erase and restore my affected dive.Obiously, it looks like an El Capitan software issue that could be a harbinger of hardware issues. Any ideas?

PS. Are there any Cloud based services where I could back up 6TB for a fee and restore from there? Of course this would take ages. I could also look for refurbished multi-TB drives. Boils down to these two perennial factors: time (which I moderately have) and /or money (which I don't have) , who would've thunk it?...

2016-01-16 18:52:29 -0600: Disk Utility started.

2016-01-16 18:52:59 -0600: Verifying volume “BIGMAC”

2016-01-16 18:52:59 -0600: Starting verification tool:

2016-01-16 18:53:01 -0600: Checking file system2016-01-16 18:53:01 -0600: Checking Journaled HFS Plus volume.

2016-01-16 18:53:01 -0600: Checking extents overflow file.

2016-01-16 18:53:01 -0600: Checking catalog file.

2016-01-16 18:54:21 -0600: Checking multi-linked files.

2016-01-16 18:54:27 -0600: Checking catalog hierarchy.

2016-01-16 18:56:11 -0600: Checking extended attributes file.

2016-01-16 18:56:29 -0600: Checking volume bitmap.

2016-01-16 18:56:32 -0600: Checking volume information.

2016-01-16 18:56:32 -0600: The volume BIGMAC appears to be OK.

2016-01-16 18:56:32 -0600: Repair tool completed:

2016-01-16 18:56:32 -0600:

2016-01-16 18:56:32 -0600:

===== Saturday, January 16, 2016 6:58:44 PM Central Standard Time =====

I did save the installer and tried to use it. The install did not work, again "Volume contains an OS X installation which may be damaged. Quit the installer to restart the computer and try again". I'll try again and will let you know. Thanks a million.

I Am at my wits' end... I erased any copy of OS Capitan installer I had, ckecked my drives, repaired permissions on my internal drive and verified/ repaired my external drive where I want to install Capitan, which initially was installed on it and ran without any problem until recently. Downloaded Capitan installer from Apple Store, which did not take long, launched the installer which gave the message that additional components were being downloaded. This took almost 17 hours... After the download was completed and I was supposed to have a fresh installer, tried again to install Capitan on my external drive and, almost at the end of a looong install process, got the same message informing that OS X could not be installed. Volume contains An OS X installation which may be damaged. Quit the installer to restart the computer and try again.

Started again, in recovery, used disk utilities to check my external drive, disk OK. Basically, no matter what I try, I cannot install OS Capitan on my LaCie 6 TB external drive. When I use my iMac 1TB internal drive with Mountain Lion, all is OK and the external 6TB LaCie mounts on the desktop as an external drive, with all data available. Guess I will have to find a multi TB drive, backup, erase and restore. Did not even try to contact Apple fot' assist. First, the moment they pick up the phone, the $ counter starts to whirr. Second, I liked them better when they were the underdog from which I bought my first Mac in 1984, not the trillion dollar, indifferent and arrogant behemoth they have becme now. As to the "rest of us" for whom the Macs were purported to be, how does one spell "sucker" in Apple-ese?

Persisrence is my middle name... Tried again. Started the installer, was requested to validate my App store access with my Apple ID, started the install again which stopped because of an "error" during installation, had to quit and relaunch the installer, message saying "downloading additional components. Your computer will restart automatically". "About 44 hours and 5 minutes remaining"... Persistence be darned, heck no! Thanks for nothing Apple.

Thanks leroydouglas, as you'll see in my answer to machack, this did not work out but I will try again. Best.

That's a definite possibility. Will download the installer from Apple Store and give it another try. The saga continues… Thanks