Four options for this annoying problem:
1. Get the firmware updated to a version compatible with Apple's specifications.
2. Activate leacy support as descried here: http://support.apple.com/kb/HT4700
3. Use muCommander to browser SMB share. Note that you cannot mount the SMB shares this way.
4. If you have access to the NAS and you can install your own software there use workarounda as described here (for pogoplug): http://techblog.abhinavsrivastava.com/2013/01/pogoplug-with-mac-os-x-mountain-li on.html