Q: imac 27 not booting only grey screen

Hey there Metrograph,

It sounds like you are unable to get to your desktop and cannot get past the gray screen on start up. I recommend the troubleshooting in the article named:

Mac OS X: Gray screen appears during startup

http://support.apple.com/kb/ts2570

Start with these steps, as there are more in the article if you need them:

isconnect, test peripheral devices and network cables

Note: "Peripheral devices" refers to external devices other than what came with your Mac, such as hard drives, printers, or hubs that you connect via a USB or FireWire cable.

1. Shut down your Mac. If necessary, hold your Mac's power button for several seconds to force it to power down.
2. Disconnect all peripheral devices such as external hard drives or printers (leave only the display, a keyboard, and mouse connected).
3. Disconnect any Ethernet cables.
4. Start up your Mac.

• If you can start up this way, one or more of your peripheral devices (or installed software related to the device) is probably causing the gray screen issue. Connect one device, then restart your Mac to test for the issue. Repeat the process until you locate the device at issue. Make sure the device is powered if it needs to be to operate correctly. Check with the manufacturer of the device for support or possible software driver or firmware updates that may be available for the device. Try a different cable with the device if possible (such as a different USB or FireWire cable).
  
  Note: If you have multiple peripheral devices and the issue only occurs when they are all connected, but not when each device is the only peripheral connected, see the Additional Information section below.
• If you are using a desktop Mac with a third-party keyboard and/or mouse device, and the issue still occurs, try starting up with an Apple keyboard and mouse connected instead. Try starting with no keyboard and mouse connected, then connect them after start up. Also, try a different USB port on your Mac.

If the gray screen issue persists with no devices connected, go to the next section (with the peripherals still disconnected).
Perform a Safe BootSimply performing a Safe Boot may resolve this issue.

1. Shut down your Mac. If necessary, hold your Mac's power button for several seconds to force it to power down.
2. Start your Mac, then immediately hold the Shift key. This performs a Safe Boot. Advanced tip: If you want to see the status of a Safe Boot as it progresses, you can hold Shift-Command-V during start up (instead of just Shift).
   
   Note: A Safe Boot takes longer than a typical start up because it includes a disk check and other operations.
3. If your Mac starts up as expected, immediately try restarting.

If the Safe Boot does not work, or the restart after a successful Safe Boot does not work, go to the next section.

Thank you for using Apple Support Communities.

All the best,

Sterling