Ubuntu is distributed as an ISO, which is "an archive file (also known as a disc image) of an optical disc." ISO files cannot be copied directly to disc; they need to be burned in a specific way to expand/extract the image into usable files.

To burn an ISO using Mac OS X:

1. Insert a blank CD/DVD.
2. Launch Disk Utility (Applications → Utilities → Disk Utility).
3. Drag and drop the .iso file into Disk Utility's left pane (below the hard drive(s) and optical drive(s)).
4. Select the .iso file and click the "Burn" button in the toolbar.
5. Make sure that "Verify burned data" is checked (if this option is not visible, click on the blue disclosure triangle).
6. Click the "Burn" button.

That's it!

Sources

1. Ubuntu documentation: BurningIsoHowto
2. Wikipedia: ISO image

Since this was the primary volume (startup disk), the "Repair Disk" button was grayed out, and, of course, I didn't have a clue where the Mac OS X install disc is. Not sure how to fix it, I turned to Google. Turns out the repair process is quite easy:

1. Turn off the computer.
2. Restart in single-user mode by pressing the power button to turn the computer on and immediately pressing and holding the Command (Apple) key and the "s" key.
3. At the command-line prompt, type: /sbin/fsck -fy and press Return.
4. fsck will run through five "phases." If it makes any repairs, it will display ***** FILE SYSTEM WAS MODIFIED *****If you see this message, repeat step 3 until you see ** The volume (name_of_volume) appears to be OK
5. At the prompt, type reboot and press Return.

References

• Mac OS X: How to start up in single-user or verbose mode
• Resolve startup issues and perform disk maintenance with Disk Utility and fsck