When you delete a file from your computer, it doesn't simply disappear from existence—at least, not right away. Even if you immediately empty the Recycle Bin or Trash folder, all your deletion does is earmark the space that file takes up on your hard drive as vacant. Until another file or application comes along to make use of that room, the old data will continue to sit there. Which is why specialized programs can often recover deleted files for you.
But recovery isn't always a priority. If you want to securely delete sensitive files, or you're selling your old computer to another person, you need to make sure that no clever software will be able to bring your old files back from the dead. Follow this guide to make sure unwanted files disappear forever from your computer, tablet, or phone, beyond the reach of even the most determined data forensics team. Just make sure you really want to permanently erase the data before you start.
On a computer, no matter what type of machine you're using, your first move should be to delete the file from File Explorer in Windows or Finder in macOS. Then clear out whichever undelete tool your operating system uses, either the Recycle Bin or the Trash folder. That's where most people would stop and move on to another task, leaving their data vulnerable. Here's what you need to do next.
For standard hard drives
If you're using an older desktop computer or laptop, it probably has a traditional mechanical hard drive, also called an HDD. If you're unsure, a quick glance at the specs should tell you one way or another. Data on these drives is stored close together, which makes the information easier to recover.
To overcome this problem and securely remove a file or folder, you need the help of a third-party program. For Windows, the simple Eraser tool is one of the best choices, or you can try Recuva, which is billed as a file recovery program but also performs secure deletions. Both free programs work similarly: They overwrite the vacant space on your drive with random data so that the original files and folders can't be brought back.
For a Mac with a mechanical hard drive, the process depends on the age of your machine. If you're running Yosemite or an earlier version of macOS, you can open the Finder menu and choose Secure Empty Trash, an option that overwrites all the files in the Trash folder with junk data. On newer HDD Macs, we'd recommend File Shredder ($6.99) or the free Permanent Eraser. Simply point these apps to the files and folders you want to get rid of, and they will take care of the rest.
Why do you have to use different methods for different versions of macOS? Apple has removed the Secure Empty Trash option from all versions of their software from El Capitan onwards. That's because their new MacBooks now use solid state disks, or SSDs, which work differently than the older mechanical HDDs. If you're using a Windows or Mac machine with an SSD, then you'll need a distinct approach. Read on for more details.
For newer SSD drives
Most laptops, especially those made by Apple, contain newer solid state disk drives. SSD storage is becoming ubiquitous because, although it's more expensive than HDD storage, it's also much faster. These solid state disks also handle file deletions differently than mechanical drives do: They don't give you the same control over where data gets written to, so those overwrite programs we mentioned earlier won't work on solid state drives.
So, instead of writing over their deleted files, SSD users need to encrypt the disk. Encryption means the username and password you use to log on to your computer will act as an unlock code for the files on disk, deleted or not deleted. Without that code, no one can read what's on your computer or recover erased files. The only danger then is if someone discovers your username and password combination, logs into your machine, and fires up a file recovery program. Because SSDs store bits of data at random locations around the disk, you don't really have any options to prevent people from doing this. Just choose a strong password and be careful about who gets access to your computer.
Macs should automatically encrypt files, but you should make sure by opening up System Preferences, then Security & Privacy, then the FileVault tab. If it isn't already set to "on," then turn FileVault on, and it will make sure your deleted files become inaccessible. On Windows 10, you can employ the built-in BitLocker tool (search for it from the taskbar), but only if you have Windows 10 Pro. If you're on the home system instead, you can use a third-party alternative, such as the free VeraCrypt, to encrypt your disks.
If you're selling your computer, and want to permanently delete all of the files on it, you can go beyond encryption to protect your data. First, transfer everything you want to keep to another machine, remembering to back it up. Then, fully reinstall the operating system to securely wipe all the data on the SSD.
To reinstall macOS, follow Apple's instructions. For Windows, you can follow Microsoft's reinstallation instructions, or use SSD manufacturer tools to perform the full wipe as comprehensively as possible. These tools are available for Intel, Corsair, SanDisk, OCZ, and Samsung solid state drives.
A note on tablets and phones
Computers are all very well, but many of us keep vulnerable information on tablets and phones as well. These devices rely on flash storage very similar to that used in SSD drives, so the deletion principles are the same: With no easy way to securely delete files, you must encrypt the data stored on your mobile devices. On the bright side, it's virtually impossible to recover deleted files, because apps only get limited control over the file systems on smartphones and tablets. So on these devices, you don't really have to worry about files making an unwanted return.
Rather than bothering with secure deletion, you should instead protect your phones and tablets against unwanted visitors: Enable a fingerprint ID or at least a PIN code to prevent anyone but you from accessing your devices. If no one else has access, then they can't mess around and recover erased files, which would take a high level of technical know-how in any case. All iOS devices, as well as all Android devices that run version 6.0 (Marshmallow) or later, apply encryption by default. So as long as you protect your lock screen, you'll be able to protect your data as well.
That said, if you're getting rid of your mobile device, then perform a complete factory reset to securely wipe everything on it. As always, first make sure you have backups. In iOS, your next step is to hit the Reset option under in the General menu in Settings. On an Android device, open Settings and head to System, then Reset. In some cases, it is technically possible to retrieve data after such a reset, but that would require FBI levels of hacking ability, so don't lose sleep over it.
One final option we haven't mentioned is putting on a pair of safety goggles and then taking a hammer to your hard drive or smartphone and bashing it into oblivion. For good measure, you can run a few nails through the hard drive or flash storage to make sure it can never be accessed again. After that type of damage, no one will ever get your deleted files back. If you're getting rid of your computer or mobile device, physical destruction is the ultimate in paranoid data protection.