FTP transferring is denied randomly


I’ve been using ATV for a while now and really like it.
But there’s one thing I cant figure out. When I’m connected to the atv through Transmit I got no problem transferring or renaming files on atv, sometimes. But sometimes when I transfer, move or rename a map or file it says:

Could not rename: “Filename”

Server said:
Rename/move failure: Permission denied

Error -150: could not rename remote file

Why is it like that? I mean, half of the files I transfer or move on the external drive on the atv doesn’t say anyting. And i cant find any red thread in which files that i can or cannot move.

Any smart solutions?