I bought a fake 256GB USB stick. Here's how to check yours.

How a fake stick works

The stick is a small, cheap memory chip with altered settings, so your computer believes it's much bigger. You can copy files onto it and it looks fine. But once you go past the real size, new files quietly overwrite old ones. Nothing warns you. You only find out when photos or documents won't open, often months later.

The clue that gave it away

A genuine 256GB drive shows up as about 238GB on a computer, because computers count storage slightly differently from manufacturers. This stick claimed 256.8GB — more than its label. Real drives are never bigger than the label says.

How to check your own stick or SD card (free)

• On Windows: download H2testw, a free tool. Point it at the empty stick and let it write and check the whole drive.
• On Mac or Linux: use the free F3 tool (f3write and f3read, or f3probe on Linux).
• Test a new stick before you put anything on it, because a full test can overwrite what's there.

Signs a stick could be fake

• A price far below the big brands for the same size.
• No brand, or a brand you've never heard of.
• Sold by a marketplace seller rather than the shop itself.
• It shows as bigger than its label.

If yours is fake

1. Stop saving anything to it.
2. Copy off anything important straight away and check the files open.
3. Return it as "not as described". Under the Consumer Rights Act you're entitled to a refund. A photo of the test result is your evidence.
4. Report the listing to the marketplace.

I've sent mine back for a refund and ordered replacements from reputable sellers. They'll be tested the day they arrive, before any customer file goes near them.

Worried about yours?

If you're not sure whether a stick, SD card or external drive is genuine — or you've lost files from one — get in touch and I'll test it for you.