Many systems allow establishing storage hierarchies to help organize storage. For example Tivoli Storage Manager provides for pools of storage volume where one volume automatically 'overflows' into the next lower volume in the pool hierarchy when the upper pool becomes too full. That is, lesser-used files are transferred into the lower pool to free up space in the upper pool. Typically this is used where the top pool is disk storage...
and the lower pools are progressively slower storage media, such as tape.
These kinds of hierarchical architectures are very useful, but almost all of them have an important caveat. They will not support circular references. In other words, if volume A flows into volume B which flows into volume C, then volume C can't flow into volume A.
Stated that baldly, the problem is pretty obvious. However as architectures become more involved and structures such as volume pools become more elaborate, it is all to easy to violate circularity, especially when making modifications such as adding or deleting volumes.
It is usually worth taking a minute to 'desk check' such hierarchical structures to make sure you haven't introduced a circular reference before committing.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.