Cant write tags to remote NAS drive

I have my music on a DNS323 NAS drive, with Mp3Tag I can see the music and read the tags but when I try to asve and edited tag I get an error message saying " cannot open file ...... to write tag" and then 90% of the time Mp3tag crashes. I can write tags with other programs so dont think its a file permission issue.
Mp3tag is a great program and would really like to get it working.
Any help much appreciated.