1) Mount manually in proxmox
sudo mount -t cifs -o username=MyUserName -o ro //192.168.1.XX/S
[//192.168.1.14/mnt/nas/TimeMachine/smb]hareName /mnt/pve/ShareName
2) Then attach to your VM
pct set 103 -mp1 /mnt/pve/ShareName,mp=/mnt/ShareName
(note, you may