I don't how Calibre works so it's hard to guess...all I can do is list what my server looks like:
Arch Linux 64bit,
RAID1 with
ext4 file system,
Samba 4.4.3 with config:
Code:
create mask = 0664
directory mask = 0775
force create mode = 0664
force directory mode = 0775
map to guest = Bad User
force user = ***
[share]
path = ***
public = yes
available = yes
browsable = yes
writable = yes
*** - yes, those asterisks replaces something that was there :P