Error restoring Custom Firmware

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Hi everyone. I'm trying to create a custom firmware for my iPod Touch 3G, so I downloaded the original IPSW and via Sn0wbreeze I created my custom IPSW. Then I wanted to manually edit some files inside it too, so I changed its extension to ZIP and I uncompressed it. I converted the root filesystem to a Read/Write image by typing the following command in my Terminal:

hdiutil convert -format UDRW [PATH TO READ-ONLY IMAGE] -o Desktop/rw.dmg

I mounted it and I edited some very little things, for example I removed some wallpapers and ringtones. After that, I unmounted it and I converted it again to a Read-Only image by typing:

hdiutil convert -format UDZO Desktop/rw.dmg -o Desktop/ro.dmg

I renamed it and I replaced the original fs with it. I zipped all files and I changed the extension to IPSW. But, restoring with iTunes form pwnd DFU I get continuously error 14. I changed my cable and my USB port; I tried creating a new IPSW, etc. but nothing.

Please can you help me making it to work? Thank you, and sorry for my bad English (I'm Italian).

P.S. I know that I could edit the filesystem AFTER restoring the firmware, but for some reasons I need a firmware already edited.