Tente isso nas duas máquinas.
Primeiro, faça o backup do arquivo smb.conf existente:
sudo mv /etc/samba/smb.conf /etc/samba/smb.conf.old
Em seguida, coloque isso em /etc/samba/smb.conf:
[global]
server string = Your computer's description
workgroup = Your workgroup name
security = user
map to guest = Bad User
name resolve order = bcast hosts wins
client lanman auth=Yes
wins support = yes
include = /etc/samba/smbshared.conf
E então em /etc/samba/smbshared.conf coloque isso:
[Desktop] ; user="yourusername"
force user = yourusername
path = /path/to/share
writable = yes
public = yes