I jailbroke my iPad iOS 4.2.1 (tethered) and updated mobile substrate, Cydia asked me to reboot my iPad and i did. iPad stuck on Apple logo for 3-4 minutes then turned off and booted again but again didn’t surpassed the Apple logo for 3-4 minutes and cycle continues. I tried hard reboot but no luck. Then i tried “just boot tethered now” and it works like charm.
2. From the next screen, select “Just Boot Tethered Right Now” and then click Next again.
3. Follow the on screen instructions to enter in to DFU mode and your device will be boot properly in seconds.