My Macbook pro (15" from mid 2009 but upgraded to mountain lion a couple months ago) started getting really slow and unresponsive as I was using it. I shut it down manually and then started it back up but it hung on the gray screen with the apple logo and spinning circle.
Rebooting in verbose mode gave me disk0s2: I/O error
Booting using command+r and using disk utility to repair the disk didn't find anything wrong.
Additionally, I have a windows partition and can successfully boot and use that on the laptop.
It doesn't look like a hard drive failure to me because i can still use some of the partitions on the drive, but I'm not sure what else could be going on?
Does anyone have any other things I can try and debug or suggestions as to what is going on?
Did you use Disk Utility on the "Macintosh HD" partition or for the whole drive?
It still sounds as if something's up with your startup disk (partition). Try running Disk Utility again. If still no errors, try resetting SMC. No joy? The Apple Hardware Test. Still having trouble? Time for a trip to your local Apple Store or AASP.