Difference Between Unallocated and Free Space

By Dave Maddox

"Empty" computer storage can be "unallocated" or "free."
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You don't have to be a computer programmer to encounter the concepts of "unallocated" and "free" space in computer storage. Both terms refer to storage space that does not contain data, but it refers to this space in different ways. If you're trying to translate an error message from your storage or memory system, you may encounter these terms. If you're planning or modifying your computer system capacity, it may also be important to know what these data structure terms mean.

Unallocated and Free Storage Purposes

Unallocated and free space both refer to computer storage, but free space is prepared for use and unallocated space is "raw" space. Before using unallocated disk space for data on a Microsoft Windows system, it requires partitioning (allocation) as a new "drive," such as "drive F" and then formatting. It will then show as "free space" in the disk utility.

Difference Between Unallocated and Free

Consider a big bag of dinner rolls for a banquet, maybe several hundred. They are unallocated. Serve a basket of ten rolls to ten tables, those are free rolls, allocated and organized, but not spoken for.

Storage for Planning Purposes

System managers have to think about how much unallocated space to keep available for expanding disk partitions, referred to by "drive letters" in Windows and how much to make available as ready free space. Large storage devices such as Network Attached Storage and Redundant Array of Independent Disks units make these decisions more challenging, especially when implementing databases.

Structure and Forensic Concepts

Important to computer forensic investigators, unallocated and free space provide more information than you would think. Unallocated space has no overlaid "file system" which defines the files and the data space they occupy, while "free" space is part of a file system, but not occupied by data. Depending on the operating system, unallocated or even free space may contain data previously deleted, since deletion occurrs by marking the space unused without erasing it.