Determining Drive Capacity
To calculate the time required to wipe a hard drive, the drive's capacity and to a lesser extent its interface type needs to be known. The drive capacity can be retrieved using the operating system or by performing an Internet search of the drive's make and model number. The drive interface can make a difference with a Western Digital Raptor or a U320 SCSI drive taking somewhat less time than an inexpensive, consumer grade ATA drive. It should be noted that calculating these time differences is not an exact science and if this information is critical it would be best determined by hard data gathered from repeated drive wiping on exactly the same computing platform.
Choosing Wipe Method
Most software tools used for drive wiping will allow the end user to select which drive wipe method to employ and how many passes over the drive the tool should make. A cursory wipe, which will deter the casual computer user from ever retrieving data from a drive, will perform a single pass of writing zero sequentially across the drive. Moving up to a Department of Defense (DoD) method of wiping data, one that should prevent any but the most professional methods from recovering any data, involves nine passes over the drive randomly marking zeros and ones during these passes. Due to the randomly generated writing procedure utilized in these higher-level wipes, the time required to complete this operation is significantly longer per gigabyte than a single pass sequential write wipe. Please note that a single pass, wiping procedure, will not prevent a data recovery expert from being able to retrieve some portion of the data, given the necessary amount of time and resources.
While times will vary based on the factors listed above, a consumer grade, 20 gigabyte IDE hard drive, being wiped with a single pass of zeros being written to the drive, will take about an hour to complete. Using a DoD-approved level of data eradication, one which utilizes nine passes of random writes, that same 20 gigabyte hard drive will require the better part of a day to complete. In cases where a drive with a capacity of a terabyte will be wiped using the DoD-approved nine passes of randomly written ones and zeros, it is very likely that this process will take in excess of one month of 24 hours per day processing to finish this wiping procedure.