What Is The Gray Progress Bar That Shows During Startup?
The gray progress bar will sometimes appearing during startup or after waking up a Mac from sleep. If you are seeing it during startup, your Mac is probably attempting to reconcile a problem with your hard disk. If this happens repeatedly, you should make sure to backup your hard drive with Time Machine and then use Disk Utility in the OS X Installer to verify and repair your hard drive.
The gray progress sometimes appears when you wake from sleep because your laptop battery is nearly dead. This is completely normal.