The FireCore team has released an updated version of Seas0nPass, the Mac application that lets you jailbreak the Apple TV 2G untethered on iOS 4.2.1.
With this new version is now possible to jailbreak the Apple TV 2G on iOS 4.2.1, which can be now restarted without problems after release.
Download and launch Seas0nPass from this link.
NOTE :Before you proceed with the jailbreak, you will need to do the following:
How to GuideStep 1: Now select “Create IPSW” option to build your own custom jailbroken 4.2.1 firmware for Apple TV.
Step 2: If your Apple TV is not already unlocked, select "Create IPSW"
Step 3: Download the latest firmware from Apple TV and will be used to create a custom jailbroken firmware
Step 4: Connect Apple TV to your Mac using the micro USB cable and hold down the MENU and PLAY / PAUSE button for 7 seconds (DFU mode)
Step 5: iTunes will open automatically and begin the recovery process.
Step 6: Once completed, your Apple TV is now jailbroken.
You may need this: