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A disk image is a full disk copy of the data making up the partition table, file allocation tables and data partitions without regard for operating system.
A disk image should be made prior to performing any forensic analysis of the disk. Creating a disk image is important in forensics for several reasons:
1. Ensure that disk information is not inadvertantly changed during analysis.
2. By performing an original disk image and storing the original disk, it is possible to reproduce forensic test results with an exact reproduction of analysis methods on the original evidence.
3. Disk imaging will capture information invisible to the operating system in use (e.g. hidden partitions, ext3 partitions on a Windows machine, etc.)
Popular software used to create disk images includes Norton Ghost. A raw image (bit-by-bit) copy of the original media should be done using the software, which may not be the default settings on the software.