Back a few years ago, I idly wondered how long it would take to securely erase a 1 terabyte hard drive. I had no idea at the time. More recently, I gained some real-world experience with something similar to this.
Due to having (surprise surprise) Time Machine problems, I at one point decided it was necessary to zero out my Time Machine drive. Why? Because I’d heard somewhere that zeroing a drive will “map out” any bad sectors on the drive, preventing them from being used once the format is complete. An ordinary format supposedly won’t do that.
The drive in question was a four terabyte drive. It took about four days for Disk Utility to zero that sucker.
A 7-pass wipe should take about 7 times as long, which would be about 28 days. And the most secure option, which I believe uses a 35-pass wipe (don’t take my word for it, though), well…that would take about 140 days. Dividing that by four, you’d end up with about 35 days for a most-secure wipe of a terabyte drive, or about a week for a less obsessive 7-pass wipe. This is assuming that it always takes the same amount of time to do a single pass over any drive of a given volume. I imagine that is not true in reality–some drives would go faster than others, due to inherent differences in drive performance and the amount of bad sectors encountered during the process.
This effectively disproves the, “Quick! The cops are here, wipe that drive before they grab the computer 5 seconds from now!” bulltweet that we used to see in the movies.
(As for that four terabyte drive that I zeroed out: Opinions differ as to whether that process will actually map out bad sectors on the drive. I was unable to tell if it had any significant effect at all, and suspect the entire exercise may have been a waste of time. And I’m still having Time Machine problems.)