I'm using Transmission 2.03 (11030) (actually updated to 2.04 (11151)), on Mac OS X 10.6.4. I've also installed and successfully used a WD MyBookWorld NAS.
Then I went on vacation, so I took my MacBook Pro with. When I returned and plugged the MBP back into my WiFi network, it would read the MBW drive, but wouldn't allow me to write to it. I reestablished the connection as admin, rather than guest; but it still wouldn't allow Transmission to write. The torrent would start accumulating peers, then it would be downloading form 1 peer, and that's when it would give me the error: Permission denied (/Volumes/Public/Shared Videos/__filename__
Also, I've set it up so it would keep incomplete downloads in one directory, and move them to the shared directory when complete. So now Transmission is treating files that have already been downloaded, as if they had not, and is looking to download them again.
Perhaps I've missed a configuration step here? Any help would be much appreciated.