are you trying to copy directly to your source disc or to the hard drive?

if copying to the hard drive, is there actualyl enough space?
if copying to the source disc (which I assume is single layer) have you made sure that there are no settings to adjust in nero (I NEVER use it so I don't know what it does). if yo're burning form a DL to a SL disc, you'll have to compress. If you have nero, use Recode.

if none of the above is your problem (not sure...you weren't very detailed), then I don't see what the big deal is in backing up the original again however you did it the first time. Seems like a lot fewer headaches this way.