Cannot save files on NAS

I am a relatively new user of Ubuntu and have encounted a problem saving files 
on an NAS device on my LAN.

Operating system Ubuntu 9.04
Samba 2:2.2.2-1unbuntu
smbfs and every other Ubuntu supported samba application in the repositories 
are installed on the system. 

Device:  D-Link DNS-323 NAS using software version 1.04
The device is auto-mounted on LAN clinet with a command in fstab, 
The name of my local workgroup has been entered into the smb.conf file.
There is an smbcredentials file located in my home directory. 

Problem: the NAS mounts and the client can view files, can add directories and 
create a file on the NAS  but cannot edit and save a file located on the NAS 
and cannot copy a file to the NAS.

When an attempt is made to save an edited file the client receives an error 
message:
"Could not save the file /media/RAID/A test folder/A test file.txt."
"Unexpected error: Invalid argument"

This device works perfectly with Ubuntu 8.04 using exactly the same fstab 
mount command which, by the way I have changed several times as suggested by 
various people on support forums.

I contacted D-Link and they say they do not support Linux at this time but the 
problem appears to be caused by the latest version of my OS not supporting 
security standards used by D-Link and refer me to Ubuntu forums for a 
solution. 

I searched Linux forums for a solution and while others have had a similar 
problem none of the suggestions that I have received have worked on my 
system.

I have the feeling that it is cifs that will not grant or perhaps not 
recognize write permissions on the NAS files although the error 
message ""Unexpected error: Invalid argument" seems to indicate something 
else.

Has this been reported to this mailing list and can anyone please refer me to 
a possible solution to the problem?

Thanks
Mac