Welcome to CD Freaks.
To backup a copy you should just be able to do a disc copy with Nero rather than run through Shrink again.
The problem you're getting might be down to the media degrading & so giving you read errors. If you've used Ritek G04 or G05 media then this is the very likely cause.
You could try backing it up to your HDD with DVD Decrypter as an ISO image. This app has better read error handling so might overcome the problem. A google search will find it for you and it's free.