Normally internal auditors make sure that the internal books are reconciled - that there are no gaps. This is just good business practice to show upper-level management and shareholders that the company is doing due diligence.
External auditors are required when a company (at least in the US) are publicly held or are under some type of watch. They can be more or less stringent than the internal auditors but they may decide to visit some dark corners where spiders live.
Auditors, internal or external, are not perfect and can be subject to pressures by companies and external interested parties (shareholders). Arthur Anderson.