No matter which user gets the available tape drive none can be guaranteed to finish. An unsafe state implies that some sequence of events might lead to deadlock. Processes claim exclusive use of the resources they require. System may either grant or deny each user request. The system grants requests that result in safe states only.
Weaknesses requires fixed number of resources to allocate.
May not be realistic with breakdowns, repairs, etc. requires fixed number of users. Very difficult in dynamic systems.