I spoke with a customer who had a flashing folder with a question mark on her MacBook. She called Apple and they told her it was because the internal hard drive is not mounting. She wanted to see if the hard drive was actually bad, needed to be replaced or had a corrupted OS problem. I suggested to her to use Drive Genius 3 on a flash drive to startup from and use Drive Genius 3 to run a scan on the internal hard drive to see if the hard drive has bad blocks. This would then give her more information about the condition of the drive. I guided her through the steps on creating a USB flash drive with Drive Genius 3 on a different Mac. She then could use this flash drive as a secondary boot device after her Macbook was fixed. She was thankful for the help and proceeded to run her scan.
Bootable USB steps.