slack space (file slack space)

Slack space is the leftover storage that exists on a computer’s hard disk drive when a computer file does not need all the space it has been allocated by the operating system. The examination of slack space is an important aspect of computer forensics.

To understand why slack space plays an important role in E-discovery, one must first understand how data is stored on computers that have hard disk drives. Computers with hard disk drives store data in a sealed unit that contains a stack of circular, spinning disks called platters. Each platter is composed of logically defined spaces called sectors and by default, most operating system (OS) sectors are configured to hold no more than 512 bytes of data. If a text file that is 400 bytes is saved to disk, the sector will have 112 bytes of extra space left over. When the computer’s hard drive is brand new, the space in a sector that is not used – the slack space – is blank, but that changes as the computer gets used.

When a file is deleted, the operating system doesn't erase the file, it simply makes the sector the file occupied available for reallocation. Should a new file that is only 200 bytes be allocated to the original sector, the sector’s slack space will now contain 200 bytes of leftover data from the first file in addition to the original 112 bytes of extra space. That leftover data, which is called latent data or ambient data, can provide investigators with clues as to prior uses of the computer in question as well as leads for further inquiries. In 2016, for example, the Federal Bureau of Investigation (FBI) revealed that it had reviewed millions of e-mail fragments that resided in the slack space of former Secretary of State Hillary Clinton’s personal servers in order to determine whether or not the servers have improperly stored or transmitted classified information.

Technically, a file’s slack space is the difference between its logical and physical size. The logical size of a file is determined by the file’s actual size and is measured in bytes. The physical size of a file is determined by the number of sectors that are allocated to the file. In most operating systems, including Windows, sectors are clustered in groups of four by default which means that each cluster has 2,048 bytes.

The logical size of the blue file below is 1280 bytes. This file was allocated a cluster of four 512-byte sectors, which means the physical size of the file is 2,048 bytes. The difference between 2,048 and 1,280 is 768, which means that the blue file’s slack space is 768 bytes.