I tried installing Yosemite from 10.6.8 and the install failed. Ran a check and disk needs repai but "repair disk" isn't an option (greyed out). Can't start up under old 10.6.8 because it says "could not gather enough information." Help, please!

Question

startup disk fail

Answer 1

Not lame at all. It's annoying that in order to salvage a disk the "official" way is reformat it. It's just a lack of documentation.
Diskwarrior is actually a software from Alsoft. www.alsoft.com
It's been around for ages and usually it's the last resort when everything else failes.
If you don't have the software, you could try one option.
Restart the malfunctioning computer and press T when you hear the startup sound. This will start the computer in transfer mode. You will see a huge firewire icon on the screen that just floats there.
Connect the computer to a healthy mac with a firewire cable and it should appear as a HD, just as an external hard drive would. (don't worry if it doesn´t)
Open Disk utility and in the left pane the malfunctioned Hard drive should appear. Run disk repair on the HD if possible. Once the repair disk has finished, with our without errors, open a new finder window and select the external hard drive.
You could backup the data on the Drive at this stage onto the healthy computer... just to be safe.
On the root of the Malfunctioned Disk there should be a folder named "OS X install data" or something like that. Basically something that normally would not be there...
Delete that folder.
Run Disk repair once again in Disk Utility. If successful, reboot the machine and see what happens.
If that does not work at all, then perhaps you should consider purchasing Disk Warrior. Run the same process as before, connecting the malfunctioning mac via firefire, but this time run Diskwarrior on the healthy machine, locate the malfunctioning HD and select Rebuild for that Hard drive. It will take a little while but once its done processing it will ask you if you want to copy the new build, select yes. That did it for me at least.
I hope that helps. You could check the app store or search online for software that is similar to Diskwarrior, there's probably some app that does the same thing. The only reason I used Diskwarrior is it has proven a good utility for Mac breakdowns and I've used (rarely though) it for years.