Since installing 10.6.3, a number of people have complained about limited connectivity when using Windows file sharing (SMB or Samba) over the network. This has resulted in permissions errors being presented when trying to copy files to NAS drives that are mounted via SMB.
Though browsing files on the NAS seems to work with no problem, when copying new files to the NAS an error stating "The operation can't be completed because you don't have permission to access some of the items" may show up. Despite this occurrence, when alternative methods of copying are used (such as using Terminal commands) the system will copy the files just fine.
This indicates one possible problem being with the Finder and how it is interacting with permission settings on the NAS. Here are some possibilities for clearing this problem:
Reset the NAS permissions if possible
Most NAS devices set up local accounts for administration. If your NAS has any advanced settings for permissions and access to shared folders, try resetting these permissions for shared folders you are trying to access.
Reset and recreate keychains
Open "Keychain Access" and remove the keychain entry for the NAS device. Then connect and supply your NAS account credentials again.
Reset the NAS and File Sharing
Unmount the NAS, disconnect it, and turn it off. Allow it to remain off and go to the Sharing system preferences on your Mac. Select "File Sharing," click the "Options..." button, and uncheck the option to share with SMB. Close the window and uncheck the "File Sharing" option in the main preferences window, and then reverse this process to turn sharing back on. Then boot up and reconnect your NAS device.
Clear Finder preferences and caches
Obtain a cleaning utility such as OnyX and then reboot your computer into Safe Mode. Then run the utility's cleaning operation for caches and other temporary files, and also locate and remove the Finder's preference file (called "com.apple.Finder.plist" and located in the /username/Library/Preferences/ folder). Restart the system and try the connection again.
Lastly, you can test the NAS accounts by creating a new one on the device to see if you can properly copy files to it. If so, then as a last resort you can try fully resetting accounts on the NAS. This will probably delete all files on the device so be sure to back them up properly first (you should still be able to copy files from the NAS, just not to it). Once the files have been backed up, delete the NAS account, even format and reset it if that is an option, and set up your account again and copy your files back to it.