DISK CLEARING

1/9/2004

Disk Clearing -- removing existing data structures from a disk -- is often a requirement when disk drives are taken out of service or have become unmaintainable. Approaches may range from simply reformatting a single partition to overwriting the entire disk with random numbers repeatedly, to destruction of the media.

Many disks are, or at least can be, divided into logical subsets called partitions. Typically floppy disks have a single partition and no partition definitions. Hard disks usually have a partition table defining the disk partitioning. Each partition looks like a complete, autonomous disk. Classically, the record(s) defining the partitions have not been part of any partition although that will change in some future disk layouts.

Simply altering the partition table can prevent access to most data, but in the case of sensitive data one must be concerned about the possibility that the data can be recovered by restoring the partition table.

In most cases individual partitions (disks in the case of floppies) can be brought to an empty state by running a formatting program. Formatting is satisfactory for returning disk space to service. It does not necessarily remove the old data and is often not a satisfactory way to erase sensitive data. For example, MSDOS has an UNFORMAT program that can recover most data from accidentally formatted FAT16 drives.

In general, fast formatting options will not remove data beyond the ability of simple software to recover it. Slower options/low level formatting programs can remove the data beyond the ability of anything other than a sophisticated laboratory to remove it. However, modern hard drives often ignore attempts at low level formatting in order to preserve embedded track id "servos" That precludes using formatting software to clear the drive if potential recovery of previous data is not acceptable.

Completely overwriting the disk data areas three times with random numbers is generally believed to be adequate to remove data beyond any chance of recovery.

Software available for clearing all or some of a disk includes:

FORMAT and formatting options in various OSes: These can be used to erase the access to a single disk partition. In some cases difficulty may be encountered in getting past legality checks intended to prevent accidental formatting. In the case of floppy disks and some older hard drives, "low level formatting" can actually remove data, but it may be necessary to understand the disk layout and options to know if the data is actually removed.

Partition management software can be used to remove/alter partitions on partitioned drives. Access controls and legality checks can present issues. For example, if an identifier contains characters that can not be displayed/typed, it may not be possible to confirm the identification of the partition to delete. It is usually possible to simply destroy the partition table. Rewriting the partition table does not erase data, but it makes recovery quite difficult.

Conceptually at least, the Linux dd command can be used to overwrite an entire disk partition with random numbers using the /dev/rnd random number device as input. It is also possible to write scripts/batch files that copy a single file repeatedly until all disk space is used up. This may encounter problems with maximum directory sizes, etc with some OSes.

Finally, there are dedicated disk erasure programs

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