How long does it take to complete a 7-pass erase on a hard disk?
Answer: A 7-pass erase is a secure way of erasing the data on a hard disk. It overwrites the data seven times, making it impossible to recover any usable data from the drive. Apple's Disk Utility programs, as well as other hard disk utilities offer this option. Some offer 3 or 5-pass erase options, while some, including Apple's Disk Utility allow you to perform up to a 35-pass erase.
The amount of time a 7-pass erase will take depends mostly on the size of your hard drive. Because the software erases every bit on the drive, the amount of time each erase will take is directly proportional to the hard drive's size. The overall time may also be affected by the speed of the hard drive, the software program you use, and possibly the computer's processor speed.
I performed a 7-pass erase on a Western Digital My Book Studio 500GB drive, connected via Firewire 800, using Apple's Disk Utility. I began the secure erase at 12:13 AM and the process completed at 6:35 PM the same day. Since the actual formatting of the drive takes less than a minute with Apple's Disk Utility, the disk formatting time is negligible. Therefore, the 7-pass erase on the 500 gigabyte drive took 18 hours and 22 minutes (1102 total minutes).
Based on the overall time, each pass took about 2 hours and 37 minutes (157 total minutes). Dividing the number of minutes by the number of gigabytes that were erased results in 0.314 minutes per gigabyte.
Therefore, each pass takes 0.314 minutes per gigabyte.
So if you have a 250 GB drive, and perform a single pass erase, it should take roughly 78.5 minutes to complete. If you perform a 35-pass erase (which is overkill for even the most important security purposes), it will take 78.5 minutes x 35 passes, which equals 2,747.5 minutes, or 45 hours and 47 minutes. Yes, that is almost two days.
These results should help you determine how long it will take to securely erase your drive. But keep in mind that there are several possible factors can affect the speed of the erase, your results may be different than mine.