How to securely erase an SD card, and prove it
A quick format leaves everything behind. Here is how to overwrite a card and check it, on a Mac, Windows or Linux, what a home wipe can and cannot claim, and when to send cards to us instead.
Short version: formatting an SD card does not erase it. Overwriting every block and reading it all back does. On a 32 GB card that takes about an hour on a laptop, and the read-back is the part most people skip.
Why a format is not an erase
A format writes a new, empty directory over the old one. The files it listed are still on the flash, untouched, until something else is written on top of them. In September 2026 we imaged two second-hand 32 GB cards that a laptop showed as empty, with a few megabytes of housekeeping and nothing else. A free recovery tool pulled 113,247 files off one and 2,639 off the other, in minutes. The previous owners had formatted them. They had not erased them.
How to erase a card yourself
The method is the same everywhere: write zeros over the whole card, then read the whole card back and confirm every byte is zero. The second step is what turns a wipe into something you can stand behind.
Mac. Find the card’s disk number with Disk Utility or diskutil list, unmount
it, then overwrite and verify. With the card at disk4:
diskutil unmountDisk disk4
sudo dd if=/dev/zero of=/dev/rdisk4 bs=1m status=progress
sudo dd if=/dev/rdisk4 bs=1m | cmp - /dev/zero
If cmp reports only that it reached the end of the card, every byte is zero.
Windows. Open a command prompt as administrator, run diskpart, list disk,
select disk N for the card, then clean all. That writes zeros across the card.
Windows has no built-in read-back, so verify with a tool such as h2testw, which
writes and reads test files across the full capacity.
Linux. With the card at /dev/sdX and unmounted:
sudo dd if=/dev/zero of=/dev/sdX bs=1M status=progress
sudo cmp /dev/sdX /dev/zero
Our measured figures for a 32 GB consumer card on a laptop’s built-in reader: 31 minutes to overwrite, 31 minutes to read back, both at 17 MB/s. A fast industrial card is quicker; a slow one is not.
One thing to know before you image or inspect a card for evidence: a Mac writes its own index files onto any card the moment it mounts it, so a card that has been in a Mac is no longer untouched. Erase first, inspect second, or work from a copy.
What a home wipe can claim, and what it cannot
An overwrite that has been read back is what the sanitisation standard NIST 800-88 calls Clear. It is a genuine erasure of every block the card lets you address.
It is not Purge. Flash controllers keep spare blocks and remap worn ones, and an overwrite from the outside never reaches those. Purge needs the controller’s own erase command followed by a verified blank read-back, and it is only claimable when that read-back proves it. Consumer cards often acknowledge the command and merely mark the blocks as discarded.
It is also not evidence. A progress bar is not a record. If someone will ever ask you to show that a card was erased, you need the card’s own serial number from its registers, the verification result, the date, and who did it, kept somewhere.
When to send cards to us
When there are more than a handful, when you need a certificate that names each card and shows the verification, when the cards must be handled under a chain of custody, or when the card will not read on your machine at all. Our line does the overwrite and the read-back in parallel across many cards, attempts the controller-level Purge and records honestly which level was reached, and issues a certificate per card. Where a card cannot be erased with evidence, it is destroyed with evidence instead.
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