Are you making a copy of a copy or multiple copies of one. If multiple copies with DVD Decrypter, once you creat a iso file you can brun as many copies as you want. I do not use Nero , so I can not say.
Assuming that your first copy is OK, the Copy Disk function in Nero should make a perfect backup. Make sure you are using quality media.
The copy plays on a stand alone fine. I do have a shitty comp' 20 gig hdd with about 6 gig free is that a problem. I have had success with this program , but no more.
6GB free space could be very problematic. 8 to 10 GB recommended - you need space for temp files and system processes. Your current availablle space is very marginal and could cause problems.
6GB free would almost force you to copy on the fly which usually isn't recomended unless you have a fast computer. I have a moderately fast computer and I usually don't do it. What you probably should do is invest in a larger hard drive. They seem to be getting cheaper every day. I have seen Hard drives in the 80GB range at fairly reasonable prices. Add that to what you have and you'ld have 100GB.