I'll try to give you some background info first.
-Me = Noob
-Installed debian (KDE 3.5.onto a Toshiba laptop
-1 user , other than root
-installed ktorrent (2.2.4) with apt-get
-configured a DNS-323 NAS unit to mount into /user/home/mirror
-Tried to get ktorrent to download directly onto the NAS which I think it will do when I fix my problem.

Now the error:
Any time I start a torrent I get an error
"[torrent name] has been stopped with the following error: cannot open /home/user/stuff/tor0/cache : permission denied"

Now if I try repeatedly to start the torrent, sometimes it works, twice total maybe. So from what I can tell this is probably something to do with my permissions in debian. But hopefully someone can tell me for sure. Thanks for any info that I can use to fix this.

/home/user/stuff/tor0/cache is probably a symlink pointing to the NAS, so you probably haven't got write access to the NAS.

You will need to mess around in /etc/fstab, Probably the best way to do this, is make a group for the NAS and then in /etc/fstab make sure that group is used when the NAS is mounted (gid option), and then make your own user part of this group.

Worked like a charm, just ended up using uid= instead of gid=.

Related question, the laptop HD is only 7GB how much space will the temp folder need? I do download larger torrents (up to 25GB) Should I move the temp folder to the NAS, or will everything be ok with the temp folder as is?

Currently I'm running a 350MB and a 13.5GB download, my temp folder is 2 MB so I'm thinking that everything is A-ok. Just let me know if I should change anything. Thanks again

7 GB is more then enough, if the temp folder ever fills your hard disk, then there is a bug in KT. The biggest thing stored there are partially downloaded border chunks, in theory that could get big, if you have for example 4 MB chunks and you are downloading 20 border chunks of those at the same time, and you then exit KT. Which would then result in an 80 MB file, but this is not a very common situation.