NASLite Network Attached Storage

Two user with OSX 10.4 and 10.6.8 are connected to NASLite M2 server. They can move-copy etc files from their computer to the server with no problem. But when working on images from the server using Photoshop they can not save the file as it gets locked. When they unlock the files and close the window it gets back to locked status and the problem is back. They use NFS and have four drives where Disk-0 and Disk-2 mirrors to Disk-1 and Disk-3. All runs fine but they can not work with the files directly from the server. They have to copy to the desktop and work with the files and then copy back to the server. It is not the way they wanted and asked me if I could help. I have no clues so far and hope maybe experts on this forum can give me hints where to look. All drives are set to "RW" in the administration window.

Thanks
Soren