How do I make sure an old hard drive cannot be read?

When erasing is enough, when destruction is warranted, and the safe way to do each — including what to do with a drive from a computer that no longer starts.

Difficulty: intermediate
Time: 45 min
Read: 3 min
Safety: caution

Short answer

If the drive is encrypted, erasing the encryption key makes the contents unreadable and that is genuinely sufficient. If it is not encrypted and still works, use a full erase rather than a quick format. If it does not work, physical destruction is the only certainty — then recycle it properly.

Deleting files and formatting a drive both leave the data in place; only the index changes. Whether that matters depends entirely on whether the drive was encrypted, which is the question to answer first.

Safety

Physically destroying a drive produces sharp fragments and, on mechanical drives, strong magnets and glass platters that shatter. Wear eye protection and gloves, and never drill or hammer a solid-state drive or a battery-containing device. Take drives to a recycling point rather than a household bin.

The order to wipe a device before it leaves the house

Step by step

Establish whether the drive is encrypted.Most modern laptops and all modern phones encrypt by default. If it is encrypted, erasing the key is what modern factory resets do, and the remaining data is unreadable without it.
Use the built-in secure erase for a working drive.Both Windows and macOS offer a full erase that writes over the drive rather than just rewriting the index. It takes hours on a large mechanical drive; let it finish.
Do not use overwrite tools on an SSD.Solid-state drives manage their own storage internally, so repeated overwriting adds wear without guaranteeing coverage. Use the manufacturer's secure erase function, or rely on the encryption key erase.
Handle a dead computer differently.If the machine will not start, remove the drive rather than sending it away intact. Laptops usually give access through one panel; a repair shop will remove it for a small fee.
Keep the removed drive if it might still hold something useful.A drive from a dead computer often still reads perfectly in an external caddy, which is also how you get the files back.
Destroy only when erasing is impossible.For a mechanical drive, opening the case and damaging the platters is definitive. For an SSD, destroying the memory chips is what matters. Eye protection, gloves, and never over a carpet.
Use a service for anything sensitive in quantity.Commercial data destruction services issue a certificate and handle recycling. For a household this is rarely necessary; for business records it may be required.
Recycle rather than bin.Drives are electrical waste. Household waste sites take them, most retailers offer take-back, and lithium batteries in any attached device must never go in a household bin.

Tips

Encrypting a drive when you first use it makes disposal trivial years later, because destroying the key destroys the data.
A drive that has held banking or medical records is worth more caution than one that held photographs. Match effort to content.
Label removed drives with the machine they came from. A drawer of unlabelled drives is its own problem later.

Common mistakes

Assuming a quick format erases the data — It rewrites the index and leaves the contents recoverable with ordinary free software. Only a full erase, an encryption key erase, or destruction actually removes it.
Putting a drill or hammer through an SSD or a phone — Fragments are sharp and any battery involved can catch fire. Erase electronically where possible, and use a recycling route for the rest.

If it doesn't work

Drive is not recognised so it cannot be erased

Cause: Controller failure, or a bad cable or caddy — Fix: Try another cable, port and enclosure first. If it is genuinely dead, physical destruction is the practical answer.

Full erase is taking many hours

Cause: Normal for a large mechanical drive — Fix: Leave it running overnight, plugged into mains power. Interrupting it leaves the job half done.

Not sure whether the laptop was encrypted

Cause: Encryption is often on by default but not obvious — Fix: If the machine still starts, check the encryption status in settings before erasing. If you cannot check, do a full erase rather than a quick one.

Questions people ask

Is a factory reset enough for a phone?

Yes, on any modern phone, because the storage is encrypted and the reset destroys the key. Sign out of the manufacturer account first, or the phone stays activation-locked.

Do magnets erase a hard drive?

Not with a household magnet. Proper degaussing uses industrial equipment. Fridge magnets and speaker magnets do nothing useful.