With the rapid development of cloud computing, cloud storage services are being widely adopted by the public, aiming to provide high-quality services to cloud users. This has become a framework that replaces traditional computing. However, as data replication and migration place data beyond users’ direct control, such data cannot always be considered trustworthy. Therefore, ensuring the integrity and reliability of stored data in the cloud is a considerably challenging task. Many researchers are dedicated to solving this problem by using third-party auditor (TPA) to verify the correctness of data storage in the cloud. Among these, multi-cloud auditing solutions are a key focus, designed to prevent irreversible losses caused by the failure of a single cloud server. This paper provides a comprehensive survey of multi-cloud storage services and analyzes and summarizes these methods, highlighting existing issues and challenges while discussing some future research directions.