I was trying to upgrade my iPhone 3G (ios 4.1 baseband 6.15) from 4.1 to 4.2.1 via itune (my think it was 10.X). However, it shown error 1015, and the iphone stuck in recovery mode. I tried to run redsnow 9.15b3 to get it out of the recovery mode, but it pops "usb communication problem". I tried again with another usb cable and on another laptop, and it still shows the same error messages. Any idea? Thank you very much.