This disk cannot be used to startup your computer?!?

Is it possible the full error message was: OS X- Cannot install on a volume used by Time Machine for backups?

The same issue on a macMini (2010)... No Solution found so far. On My MacBookPro the Update works. It's definitly a different Issue than the OS X- Cannot install on a volume used by Time Machine for backups? mentioned by Kappy.

I have dual boot with Ubuntu. Maybe this is causing the Problem.

Sounds the same as this Lion article: OS X Lion: Installer reports "This disk cannot be used to start up your computer"

If Apple's posted a Mountain Lion version, I don't see it.

Having the same problem here. It installed fine in the Mac Pro but same message on the laptop when installing: the disk cannot be used to startup your computer...

Have you tried the procedure in OS X Lion: Installer reports "This disk cannot be used to start up your computer"?

In my case, using the solution provided by Pondini worked for my MBP (http://support.apple.com/kb/TS3926).

I have this same proble. The genuises at apple told me to run Disk repair permissions. i Tried and still it does not sort the problem!!!

The link Pondini supplied helped me install ML on my MBP. A simple partition resize was the trick. Takes 30 seconds and you're good to go.

http://support.apple.com/kb/TS3926

I managed to install ML:

It was not very consumer friendly 😠.

1. boot into Recovery Mode
2. Repair Partition Scheme (Disk) via Disk Utility BE CAREFUL...
   -> This messed up my Partition table (Partition type for EFI and Recovery HD changed) !!!
3. Repair Volume "Macintosh HD"
4. Reboot.
5. Use gdisk to fix the partition table manually (Change type for EFI to "EFI System" and Recovery HD to "Apple boot". gdisk can harm your data. DO THIS ONLY IF YOU KNOW WHAT YOU DO!
6. Reboot.
7. Install ML.

This is an incredibly unfortunate problem. I am guessing it may have something to do with Bootcamp and/or rEFIt on my startup disk. Perhaps not, it might be something entirely unrelated and I am merely blaming myself instead of the installer.

Either way, I have not been able to correct the problem with any of the methods I've seen suggested here or elsewhere and am forced to reformat the disk due to lack of patience and/or gain of hostility.

Hopefully my fresh Time Machine backup will allow for a friendly migration once the clean Mountain Lion install is complete.

This is an incredibly unfortunate problem.

It seems that my Mountain Lion update issues were likely the result of two Linux partitions which completed my install disk.

After backing up, the clean install went flawlessly and a migration got me back to perfect familiarity within an hour or so.

I still find this issue disconcerting, something I wouldn't have found closely acceptable five years ago, but Apple is moving on in bizarre directions: so a user must adapt.

All in all: all is well.

Message was edited by: dereism due to a typo

Thanks for replying stefan_ch. I followed what you said basically and now have the mountain lion partition updated. I'm working on linux now. It seems the recovery partition "partition type" was critical (which you mentioned).

I posted the results over here, for those interested:

http://apple.stackexchange.com/questions/58043/upgrade-to-mountain-lion-from-dua l-boot-ubuntu-configuration/58064#58064