I always get the Apple logo and my device doesn't boot.
There are various reasons for this.
After downgrade to iOS 4.1 or lower
Maybe you have downgraded your iPhone and the hosts file is still pointing to the Cydia server. Then this means that your baseband could not get overwritten and you get in iTunes error 1013 or similar. In this case, you just need to kick your device out of recovery: Start TinyUmbrella, select your device and press the button "Exit Recovery". There are several other tools for this also.
With 4.2.1 redsn0w boot tethered
With a redsn0w tethered jailbreak, maybe yor forgot to boot tethered. This means that you have to connect your device to your computer every time you want to boot it. Start redsn0w tool, select your installed firmware and then set the option "just boot tethered right now".
With firmware 4.2.1
After installation of 4.2.1 without updating the baseband (because you have an iPhone 3G or 3GS with iPod baseband 6.15, or because you didn't upgrade the baseband of your iPhone 4 with TinyUmbrella method), your iPhone is in a recovery loop and the Exit Recovery methods of all common tools don't work. Run greenpois0n rc5 to jailbreak it and also to get it out of this recovery loop. The next version of TinyUmbrella will also get it out of this recovery loop.
As an alternative, you can update your baseband to the version that belongs to the firmware. Do this only if you have a factory unlocked iPhone. Close TinyUmbrella and remove all lines with "gs.apple.com" from your hosts file. Be aware that TinyUmbrella changes the hosts file back. Now you can Restore to 4.2.1 updating the baseband and you don't get an error or hanging in recovery loop. This doesn't work for those that installed the iPod baseband, as you cannot downgrade the baseband.
last edit: 2011-02-12
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