Unable to restore to 3.1.3 in Xcode organizer

I am using Xcode 3.2.1 and I accidentally updated my iPod to 4.2.1 in iTunes.

When I open Xcode > Org开发者_运维百科anizer and try to restore my iPod to 3.1.3.

http://img195.imageshack.us/i/screenshot20110318at630.png/

It fails with an error message: Unable to preflight personalized install

http://img402.imageshack.us/i/screenshot20110318at633.png/

I am using this software image:

iPod2,1_3.1.3_7E18_Restore.ipsw

And I am quite sure this image is not corrupted since it comes from pfd.apple.com and I have re-downloaded it for several times.

My iPod is a registered device in the Provisioning Portal and I have never jailbroken it before.

What do you think the problem is?

Thanks in advance.

I have searched in the apple member center developer forum, after reading some posts, what I can conclude is, unless you have previously saved you SHSH, you cannot possibly downgrade it from 4.x to 3.1.3.

Ignoring the option of buying another iPod which still has the 3.1.3 iOS installed, the only option you are left with, is to persuade yourself that most people are already using 4.x iOS and supporting 3.1.3 is not that important.