I've gotten Samba working on my Fedora server, and I can see and read files on it from my Windows 2000 box, and when I first got it working (I had a firewall problem) I could even write to it. But now when I try to drag files to it, or save them from an application, I'm getting an error that indicates a permission problem. I can't imagine what it could be, since the files and directories are all write permission for the user, and I'm the user. I can write to them as a user on the Fedora box itself--I just can't do it from the Window's client.

Anyone have any ideas what could be the problem, or how to diagnose it?

When I ran into this, is was because the user that had written the files (while logged in on the server) did not match the user that I was logged in as. This only explains problems with overwriting files (or writing new files to subdirectories not owned by the user).

Check your directories owner and permissions, and check any files. If the directory is owned by someone else you won't be able to write files unless it is chmod 777.

I think David Summers is correct. However diagnostic can be done via log files in /var/log/samba/. Note I don't use sambe on Fedora Core but on CentOS 3. But if /var/log/samba isn't there try /var/log/messages
I usually open up a terminal session, run 'tail -f /var/log/messages', and then try the failing operation again. Note that the messages file does have time stamps so you could go back and find it if you knew when.