A hash is a mathematical function that produces a unique value for a given input. A hash can be used to verify that a bit-level image copy of a hard drive is an exact clone of the original hard drive by comparing the hash values of both drives. If the hash values match, then the drives are identical. If the hash values differ, then there is some discrepancy between the drives. Inserting the hard drive on a test computer and booting the computer, recording the serial numbers of both hard drives, or comparing the file-directory listing of both hard drives are not reliable methods to verify that a bit-level image copy of a hard drive is an exact clone of the original hard drive. Reference: https://www.forensicswiki.org/wiki/Hashing