Cant Download Software Update

Hi,My Ipad Mini 1 Generation has a problem that the speakers don't work correctly and the fix is restore the iPad but i need to do that in a computer and I have the MacBookAir 2010 that only has 64GB and i have iTunes in an external hard drive Well my problem is that when i click the restore button it does all the backup and then it starts downloading the update but at the middle of the download it stops with the error

There Was a Problem Downloading the Software Update for The iPad IPAD NAME an unknown error occurred (-50).

Make sure your network setting are correct and your network connection is active, or try later. But my connection is all right I think that its that i have iTunes in a hard drive or i don't now what it is please if you can help me thanks.