I've also noticed that my HD won't spin down if I have samba running, but it will if I shutdown samba.
I have the same problem. A friend of mine who is doing this type of stuff as his work told me that the problem might not be the actual logging, but lock files that samba creates.
He suggested to create a small ram disk and configure samba to write the lock files to that ram disk. In the smb.conf he said I should modify
lock directory = /ramdisk
"ramdisk" seems to be the name how the ramdisk is named.
If everything works correct the ramdisk should have a bunch of *.tdb files.
I haven't found the time to do this but will report the result after I got around doing it.