There are lots of ways to obliterate sensitive data from of your drive: blast furnaces, degaussers (magnet field generators), sledgehammers, and secure-deletion software among them. These tools vary in effectiveness — especially as applied variously to hard drives, solid-state drives, and USB flash drives — and in the subsequent usability of the drive.
For the sake of argument (and a more interesting article), let's assume you'd like to preserve your drive's functionality. This rules out violence and degaussing, which, though wonderfully effective and perhaps therapeutic, will render a drive useless. Excluding those options leaves you with a choice between software and software-combined-with-firmware methods.
Free secure-erase utilities
You can easily erase an entire hard drive or SSD by using any of the free utilities listed below. All invoke the secure-erase (sometimes called quick-erase) functions integrated into nearly every ATA/SATA drive produced since 2001. By and large it's a great feature, but using it on older drives has some potential pitfalls, such as buggy implementations, an out-of-date BIOS, or a drive controller that won't pass along the commands. You might also need to fiddle with the ATA/IDE/AHCI settings in your BIOS, and in most cases the drive should be mounted internally.
I've never had a problem secure-erasing a hard drive, but about a year ago I did brick a Crucial M500 SSD. (A firmware problem was probably responsible for this disaster; Crucial accepted the drive for return but never told me why the hardware had gone belly-up.) An enhanced secure-erase operation overwrites a drive's housekeeping data as well as its normal user-data areas, but at least one vendor (Kingston) told me that its normal secure-erase routine does both, too. In the bad old days, running a secure-erase on some SSDs sometimes left data behind.
Depending on the controller you use (notably SandForce), a secure-erase can be cryptographic or physical. If a drive is encrypted — and some are by nature — a secure-erase operation simply deletes the encryption keys, and then regenerates them. Without the original keys, the data is useless. A physical erase involves zapping the drive's magnetic particles or NAND cells back to their default state.
To entirely avoid the danger of erasing the wrong drive in a multiple-drive system, you should power down, disconnect all of the drives except the one to be erased, and then boot from a CD or a flash drive with the utility that does the job. I learned that lesson the hard way.
Linux-based boot disc Parted Magic (formerly donationware, now free to use but $5 to download) has many features, including a file manager and a partition manager. It's handy for recovering data and operating systems, but it also has a link on its desktop to DiskEraser, a simple utility that will erase your drive or invoke the drive's own secure-erase routine. Parted Magic is basic and lightweight, and it will work with any drive. In fact, several SSD vendors recommend it — though the recommendations date from when it was completely free.
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