UPDATE
The solution is to restore in DFU mode, but you should probably try on a different machine. I tried this, and it still failed, but apparently there is a problem with the Apple servers that cause the IPSW to corrupt during download more often thannot. I had my device restored at the...
I am in the same boat. I have 2 iPad 3s, neither would take the update. One I was able to re-restore to 5.1.1, but the other is stuck in a recovery loop and fails with every attempt to upgrade or restore. I would really like to fix this myself, but I've run out of options.
Were you actually able to update to 6.1 or did you stick with your previous os? I have been trying to update 2 jailbroken iPad 3s. I keep ending up with error -1 on both of them. I have tried everything. All I can do is use Redsn0w to re-restore 5.1.1. this tells me that there is not really...