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If you're using an iPhone3GS that is currently pwned using redsn0w or PwnageTool at 3.x (or if you have the SHSH hashes to get back to that state), this topic provides a way for you to restore to a custom IPSW created by PwnageTool 4.1.2 on Mac OS X, even though you're on Windows. This is useful if you need to maintain the ultrasn0w unlock, but don't have a way to use PwnageTool 4.1.2 on a Mac OS X machine yourself.

WARNING
CAUTION
BEWARE

This is an advanced technique. Only do this if you and/or your friend are pretty good at computers. Really…otherwise you may lose your ultrasn0w unlock.

You must start this procedure from a pwned iPhone3GS at 3.x using redsn0w or PwnageTool. If you don't have that configuration, or can't get to that configuration, please stop now. You risk losing your ultrasn0w unlock. Please don't read further.

Get a friend with a Mac to download PwnageTool 4.1.2. Modify PwnageTool by dropping the two files in this zip into PwnageTool.app/Contents/Resources/FirmwareBundles/iPhone2,1_4.1_8B117.bundle/ Create a custom IPSW using that tweaked PwnageTool. Have your friend give that custom IPSW to you.

Get into recovery mode on your iPhone3GS at 3.x that was pwned using redsn0w or PwnageTool. Restore to your friends IPSW using iTunes. On Windows, you must use Shift-Restore in iTunes to be able to select the custom IPSW. This is very important!