Deleting a file doesn't remove it. It removes the pointer to it — the entry in the index that says where the data lives. The data itself stays on the drive until something else happens to overwrite that space, which might be next week or might be never.
That's why recovery software works, and it's why a computer sold with "everything deleted" can hand a stranger your tax returns.
For most everyday deletion, none of this matters. It matters in three situations: disposing of a device, disposing of a drive, and removing genuinely sensitive material.
The old advice is now often wrong
For decades the answer was "overwrite the file multiple times with random data," and file shredder tools did exactly that.
That advice was written for spinning hard drives, and it doesn't map cleanly onto modern storage.
On an SSD, the drive's controller decides where data physically goes. It moves data around for wear levelling, and it keeps spare capacity you can't address. When you overwrite a file, the controller may write the new data somewhere else entirely and mark the old location for later cleanup. You cannot reliably overwrite a specific file on an SSD, which means file shredders don't do what they claim there.
On a modern hard drive, a single overwrite pass is generally accepted as sufficient. The multi-pass approach originated with much older drive technology, and repeated passes on a current drive mostly waste time.
So the modern answer depends on what you're doing.
What actually works
For a whole drive: encryption plus a wipe
The simplest reliable approach: encrypt the drive first, then erase it.
If the drive was encrypted while in use, wiping the encryption key makes the remaining data cryptographically meaningless. Even data the controller stashed somewhere unreachable is unreadable, because the key is gone.
This is why full disk encryption is worth turning on even if you're not worried about theft — it makes eventual disposal trivial.
On a Mac: modern Macs erase in a way that discards the encryption key when you use the built-in erase function. Erase All Content and Settings, or erase via recovery mode.
On Windows: BitLocker-encrypted drives can be wiped by removing the key; the built-in reset function has an option to clean the drive, which is the one to choose when disposing of it.
On any SSD: the drive's own secure erase command, issued through the manufacturer's utility or the system firmware, instructs the controller to wipe everything including spare areas. This is the most thorough software approach for SSDs.
On a hard drive: a single full-drive overwrite is sufficient. Both operating systems' disk utilities offer this, and it takes hours.
For individual files
On a hard drive: a file shredder tool that overwrites the file works, then empty the trash.
On an SSD: you can't reliably target a single file. The practical options are to encrypt the file so that deleting it leaves unreadable remnants, or to accept that individual secure deletion isn't achievable and rely on whole-drive measures at disposal time.
Better approach for both: keep sensitive material in an encrypted container from the start. Deleting it leaves encrypted fragments, which are useless without the key.
The physical option
For anything genuinely sensitive, or for a drive that's failed and can't be wiped: destroy it.
Failed drives are the important case. You can't run a secure erase on a drive that won't respond, and a drive that's mechanically failed may still have perfectly readable platters.
Options: drill through the platters, use a drive shredding service (many recycling centres offer this), or open it and physically destroy the platters. Bending or breaking the platters is sufficient for a hard drive; for an SSD, the memory chips themselves need to be destroyed.
Get a certificate of destruction if it's business data with a compliance requirement.
Before you sell or give away a computer
- Take a full backup, and verify you can restore from it. Do this first, before anything else.
- Sign out of every account — your platform account, cloud services, email, applications with licence tie-ins.
- Deauthorize licensed software that limits activations.
- Remove the device from your account's device list, and from find-my-device services.
- Confirm the drive was encrypted. If not, encrypt it now, then erase.
- Erase and reinstall the operating system, choosing the option that wipes rather than the quick reset.
- Check the firmware or BIOS for saved passwords or settings.
- Set it up as far as the initial welcome screen and stop, so the new owner starts fresh.
The account sign-out steps matter as much as the wipe. A device still registered to your account may be locked to it, which strands the buyer, or may leave traces of your account.
Before you sell or give away a phone
- Back up, and verify.
- Transfer authenticator codes to your new device and confirm they work.
- Sign out of your platform account — this is essential, as activation locks otherwise leave the device unusable for the next owner.
- Remove the SIM and any memory card.
- Factory reset.
- Confirm the device no longer appears in your account's device list.
Phones are encrypted by default when a passcode is set, so a factory reset on a passcode-protected phone is generally sufficient. The account sign-out is the step people forget.
Other places sensitive data hides
Wiping the main drive isn't the whole job:
Printers and copiers. Many have internal drives storing everything printed or scanned. Business machines in particular. Check the manufacturer's disposal guidance.
Routers and network devices. Store wifi passwords, network configuration, and sometimes logs. Factory reset before disposal.
External and USB drives. Old backup drives, memory cards from cameras, USB sticks in a drawer. Each is a complete snapshot of something. Wipe or destroy.
Old phones and tablets in a drawer. Common, and each holds photos, messages, and saved credentials.
Cloud accounts you've abandoned. Deleting local copies doesn't touch them. Close the account properly, and note that providers typically retain data for a period after closure.
Backups of the thing you're wiping. If you securely delete a file, copies may exist in your backups, in version history, in cloud trash, and on an off-site drive. For genuinely sensitive material, track down every copy.
That last one is the most commonly missed. Secure deletion of an original is undone by a backup nobody thought about.
When this actually matters
Be proportionate. For most files, ordinary deletion is fine — the risk of someone running forensic recovery on your drive to find your grocery list is not a real risk.
It matters when:
- Disposing of any device or drive, always
- You handle other people's personal data and have deletion obligations
- You're in a regulated profession with specific requirements
- The material would cause real harm if recovered — financial details, health records, legal documents, identity documents
- A device is leaving your control for repair, resale, or recycling
For everything else, the practical protection is full disk encryption plus normal deletion. Encryption is what makes ordinary deletion good enough, which is the underlying argument for turning it on everywhere.
The rule that simplifies all of this
Encrypt everything from the start, and disposal becomes easy.
An encrypted drive that's erased is unreadable. An encrypted file that's deleted leaves unreadable fragments. An encrypted backup drive that's lost is a lost drive, not a data breach.
Doing the encryption up front removes almost every difficult decision at the other end.
Secure erase methods, tool availability, and platform behaviour vary and change. For regulated data, follow your sector's specific requirements rather than general guidance.
Next in this series: How to Spot a Phishing Email Before You Click