One of my relative’s iPad2 got stuck in the iTunes logo interface and stayed unusable for a while. Just tried to get it restored by iTunes via USB. However, it failed, always auto rebooted after progress bar showed under the Apple logo.

Tried with DFU mode and restore process still failed with “Unknown Error 9”. This thread googled suggests about the unbelievable culprit: the latest iOS 9.2.1 is incompatible to restore iPad2.

Since older iOS version cannot be signed and updated by Apple now, latest iOS 9.3 beta 4 works fine to restore iPad2 successfully.

Unbelievable.