A forensic image (forensic copy) is a bit-by-bit, sector-by-sector direct copy of a physical storage device, including all files, folders and unallocated, free and slack space. Forensic images include not only all the files visible to the operating system but also deleted files and pieces of files left in the slack and free space.
Forensic imaging is one element of computer forensics, which is the application of computer investigation and analysis techniques to gather evidence suitable for presentation in a court of law.
Not all imaging and backup software create forensic images. Windows backup, for example, creates image backups that are not complete copies of the physical device. Forensic images can be created through specialized forensic software. Some disk imaging utilities not marketed for forensic use also make complete disk images.
In the case of cybercrime, additional evidence may be discovered other than what is available through an operating system in the form of incriminating data that has been deleted to prevent discovery. Unless the data is deleted securely and overwritten, it can often be recovered with forensic or file recovery software.
Creating and backing up a forensic image helps prevent loss of data due to original drive failures. The loss of data as evidence can be detrimental to legal cases. Forensic imaging can also prevent the loss of critical files in general.