Q When I tried to install Ubuntu on the Boot Camp partition of my 27” iMac, it crashed, and restarting complained it could not find an operating system. Restarting with the Option key held down, my startup disk had been trashed, and it could not start up from its OS X install disk. How can I get it set up again?

A There are four possible causes for your iMac failing to start up from a bootable optical disk, such as an OS X Install disk:

• the disk could contain a release of OS X that is earlier than the earliest that is compatible with your Mac;
• the disk could be damaged;
• the optical drive in your Mac could be damaged;
• or your Mac’s firmware could be corrupted.

Your iMac 11,1 model was introduced in October 2009, and probably shipped with Mac OS X 10.6.2. The standard release of Mac OS X 10.6.0, as would be provided in an earlier upgrade set, lacks the hardware drivers necessary to support your iMac, so the first thing to check is that you have the correct (or later) install disk. If you cannot find the original, a current retail release should work, provided that it is of 10.6.3 or later.

Inspect the optical disk for scratches or other damage that could be preventing it from working.

If those are correct but you still have no joy, try a gentle optical drive cleaner intended for computer use. Avoid those that use any more than a trace of liquid, so that you do not risk flooding the inside of your iMac.

The final cause, firmware damage, is unlikely, as this model had not had any firmware updates. For those that have, Apple provides firmware restore packages which are detailed in this article.

Updated from the original, which was first published in MacUser volume 27 issue 11, 2011.