Samba share randomly unable to move files/folders
After a bit of research I've come up with a workable solution - at least I'm not changing it for the time being.
Check 'ignore ownership' is unticked on the external volume where the share resides.

Change the users primary group id from 'staff' to custom group id

sudo dscl . -change /Users/lmessi PrimaryGroupID 20 567


Remove all ACLs from External drive 'Disk1'

chmod -RN /Volumes/Disk1


[if you get any errors, try recursive unlock files beforehand]

sudo chflags -R nouchg /Volumes/Disk1


change group ownership recursively for the group 'design'

sudo chgrp -R design /Volumes/Disk1


You can check ownership with ls -gl

ls -gl /Volumes/Disk1


Then my smb.conf file looks something like this

[global]
    socket options = TCP_NODELAY IPTOS_LOWDELAY SO_RCVBUF=65536 SO_SNDBUF=65536
    unix extensions = no

        guest ok = No
        read only = No
        force group = design

    create mask = 664
        force create mode = 664
        security mask = 664
        force security mode = 664
        directory mask = 2775
        force directory mode = 2775
        directory security mask = 2775
        force directory security mode = 2775

[Share1]
        comment = Share1
    path = /Volumes/Disk1/Share1
        valid users = mickey, donald
        admin users = mickey, donald