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https://lum7671.wordpress.com/2010/10/13/iphone-4-2-beta%ec%97%90%ec%84%9c-4-1%eb%a1%9c-downgrade-1015%ec%97%90%eb%9f%ac-%ed%95%b4%ea%b2%b0/
iPhone, 4.2 beta에서 4.1로 downgrade, 1015에러 해결