Recently I have had problems with finder not being able to copy and paste files from my network shares. I could still use quick look, preview etc.
The problem seemed to be specifically with finder although this article seemed to suggest it was todo with samba file locking Maybe finder got a bug in the last mac update and this is just a work around.
Any way adding this option seems to help finder copying files over samba (option goes in the smb.conf file on the server)
#Stop Mac Finder thinking it can not copy files
locking = no