How much hard disk cleaning is enough?
Whether you're disposing of a hard disk or simply redeploying it as a new data or system drive, the disk drive needs to be wiped of old data. This means more than erasing the disk, as erased data can be recovered. The disk should be scrubbed with a disk cleaning utility.
Using the operating system's "delete" command doesn't actually delete anything. It simply marks the space on the disk as "available" so the operating system can re-use it. This is fast, but it leaves all the data in place on the disk, easy to recover. A disk wiping program, on the other hand, actually writes over the data to make it unreadable.
Acronis Drive Cleanser, which is sold as a stand-alone product and as part of the Acronis Privacy Expert Suite, offers multiple levels of disk cleaning. The basic level writes over all the sectors on the disk once, making them unreadable by ordinary means. However, this is not a "secure" overwrite.
The second level writes all sectors repeatedly with different bit patterns, rendering the information completely unrecoverable. In fact, the user can select from multiple overwriting algorithms, including several used by the U.S. Department of Defense, as well as others from Germany and Russia. In addition, two extra and more secure algorithms are included that overwrite the data up to 35 times!
Because the basic level of scrubbing only writes over the disk once, it is fast. However an expert cracker with the right software and enough patience may be able to recover at least some of the material on the disk. Multi-pass scrubbing makes it much less likely that anything readable will be left on the disk.
Basically it comes down to the level of security you need. If the information on the disk is extremely sensitive, such as customer information or credit card numbers, the added time it takes to use a multi-pass scrubbing technique is probably appropriate.
Otherwise, if you are going to re-use the erased hard disk on your own PC, then basic cleaning should be enough. If you are going to dispose of the hard disk by donation, resale or junking, a multi-pass erase is highly recommended.