Samba triggers HDD each time a client is connected. This happens only because each such moment a socket is created under "msg.sock" dir, which is located under private dir (which obviously should be located on a persistent media).
There were a few messages about this on mail list (see  and bellow the thread).
Also it doesn't corresponds FHS well according which such stuff as sockets should go to [/var]/run/.
I would like to ask that those files would be moved away to [/var]/run/samba or whatever place which is meant for temporary runtime files (and mounted to tmpfs by most distros).