How is data stored on HDD and SSD, and how does that change the way data is securely erased from them?
In: Technology
Data stored on SSDs requires an electric charge to be maintained; over long enough periods of time without electrical power, the charge will dissipate, wiping it’s contents.
On a hard drive, data is written on magnetic particles on the disk by changing it’s magnetic characteristics to north and south, one representing a 1 and the other a 0. On hard drives, data is written to a table of contents. Deleting a file simply clears the entry in the table of contents, but the data is still on the drive. There is software you can buy to scan every sector of the drive to rebuild any deleted files that haven’t been overwritten.
Data on a hard drive is stored by actually going to the surface with a magnetic field strongly, causing it to hold onto that polarity – north or south, 0 or 1 – at that location. This would store 1 bit. Reading is also simple, detecting the magnetic field. Like a CD or record player, the surface of the disk spins and there’s something moving back and forth along the radius. When you need to read or write from a particular spot you just slide into the right position and wait for the rotation to bring the data to the mechanism. This is also the reason it’s so slow – the moving parts and waiting for things to line up – and why they tend to die after a few years… precision parts spinning at high speeds will wear out, or just be vulnerable to even minor vibrations, etc.
Solid state drives, for the most part (NAND flash is the most common type, but there are a few exceptions like Optane) have a lot of little storage “cells” that can hold a static electric charge. This is like how your body can become electrically charged if you rub your feet on the carpet while wearing socks. As long as you don’t touch anything, you hold that electric charge. The drive either puts in a charge, or it does not, into a cell and it floats there as a 0 or a 1. (New drives hold a variable voltage in the cells so that 1 cell could hold more than 1 bit of data, but I’m going to ignore that). There are no moving parts. However an important part of how these work is that there is a discrete “erase” operation which actually does physical damage to the cells over time, but resets them back to no-charge. The erase process is the slowest action on an SSD, and is why SSDs do “Wear out” over time.
“Securely” erasing data on a spinning HDD requires actually going over the whole thing and writing out the 0 bit across the entire surface. While the drive does this for you, the process still typically takes hours.
Whereas with an SSD, the Erase operation is performed on the entire disk as fast as possible. Each physical chip on the SSD storing data can erase itself at the same time, and the process is relatively quick… maybe a minute or two?
The words “secure erase” means all the data must be gone and not recoverable. A full wipe is necessary, and why it takes so long.
High end “enterprise” drives offer a 3rd option. The contents of the disk are encrypted and the drive itself handles the encryption work, making it look like a normal hard drive but you have to give it a password as the computer starts. Well, the password is not DIRECTLY used to encrypt the drive since it’s possible to change the password without rewriting the whole surface to accommodate. The password just unlocks the real encryption key saved on the disk, and so a password change just affects this little key slot. “Secure erase” then simply wipes out the key slot, rendering the data unreadable without the key and the password doesn’t work any more. Mission complete. This is available for both HDDs and SSDs, but they aren’t the kind of drives home users buy.
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