We’ve come across a couple of iPads that give you an error 9 when you try to restore them to factory settings, usually after the iPad gets disabled or has some other funky state that cannot be recovered.
Apple has its own official help page for errors, but we didn’t find that much use (no, security software wasn’t blocking anything).
In one case, the iPad hadn’t been updated in a long time and wasn’t able to jump up versions to 9.2 from whatever it was (more details on this type of situation at iOS 4.3.5 error 9 when updating to 9.0.2), and Apple stopped signing older .ipsw files, so you can’t even manually download those and use those as intermediate updates to the final one.
In another case, it was simply a bad USB cable… or a perfectly fine USB cable for charging and general file transfers but apparently not good enough to do a factory restore on. I’d say for this type of recovery it’s worth trying to find the newest, shiniest iPad/iPhone cable you can find.