In this paper, storage clouds are online services for leasing disk storage. A new modeling tool, formulated from empirical data spanning many years, lets organizations rationally evaluate the benefit of using storage clouds versus purchasing hard disk drives. The business of selling infrastructure as a service through the Internet is growing. This technology trend, also known as cloud computing, lets individuals and organizations outsource their IT requirements to remote data centers, paying for only what they use.Several online services currently lease storage infrastructure. These storage clouds let anyone with a credit card purchase storage capacity online, paying a monthly fee for the storage they use. With the significant growth of society's storage requirements, and the availability of pay-per-use online storage services, when should a consumer consider using storage clouds? The paper focuses on the problem of resolving this buy-or-lease storage decision.