Run on the banks

If you've got money in a Greek bank, and you think there's a significant chance that Greece will leave the Euro and switch your holding into rapidly depreciating Drachmas, the prudent thing to do is to take the money out now and stash it away somewhere the Greek government can't get at it.  The seriously rich will have got their money out already, but there must be a lot of ordinary people with savings.  So why wouldn't there be a run on the Greek banks before the referendum?