From: Daniel Bird <dbird@sgul.ac.uk>

> Setting locking = No in the globals of smb.conf
> fixed it.

Keep in mind that:
"Be careful about disabling locking either globally or in a specific
service, as lack of locking may result in data corruption. You
should never need to set this parameter."

Trying again since it didn't seem to make it last time:

Well, re-exporting NFS imports via Samba is not really a supported
way to do things either, exactly due to the problems the OP is
encountering. So, if you are going to live on the edge, you may have
to get closer to it to get things to work.

This is usually an NFS issue, not a Samba one. Try setting
"posix locking = no" in smb.conf.

Ian