3 months ago i used bootcamp to installed windows 8.
Maybe sometime when i click to MAC OS Drive on win 8, they tell me need to format but i didnt.
Now i want to switch back to MAC OS but can, when reboot hold down alt/windows key, nothing happened.
How can i resolve this prob
Welcome to Apple Support Communities
Are you using a Bluetooth keyboard? If so, are you using the Apple Wireless Keyboard? If not, get a wired keyboard. Some Bluetooth keyboards don't work during start.
Another way of starting into OS X is by holding the X key while your Mac is starting. If it doesn't work, probably you removed OS X
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