Chosen Solution
When I tried to update after restoring iPhone 4s, till 60% it’s updating after that unknown error occurs
@nithinrosh -1 is usually indicative of a failed Baseband (IMEI). You do want to try a few things like: Use the back USB port on your computer (do not use a hub etc. Depends on your computer of course) Use an Apple brand charger Use an Apple brand cable Clear your computer’s DNS cache (depends on OS and computer) Restore in DFU mode Some suggestions other made is a new battery as well as using a different computer. Also, instead of trying iTunes check on 3utools for a restore.
