Strange problem when copying files to NAS

Hello all. I have a strange problem when trying to copy files from my intel iMac to my Seagate NAS. I am copying files from my HD and when the file is nearly copyied i get an error message saying that the file can't be copied because it is already in use by another application. Well... it surely isn't. I try copying them by ftp (Filezilla) and everything works fine. I can then access them through my media server but !!!! NOT THROUGH MY iMac!!!! It is like the files are not there!!!! But they definetely are! I tried fixing the disk permissions, rebooting NAS, iMac etc but nothing seems to solve the problem. Is there any suggestion of what might be wrong? I am under Mavericks and i am pretty sure that the problem started after the update. When copying from my external USB hard drive to my iMac HD everything works fine. The problem is from my iMac to my NAS.