---
title: Testing SD cards for fake capacity with f3 under WSL2
date: 2020-04-27
summary: How to mount a Windows drive letter (an SD card) in WSL2 with drvfs and run f3write/f3read on it to detect counterfeit, under-capacity cards.
tags: wsl, sd-card, f3, windows
environment: Windows 10; WSL2 (Ubuntu); f3 7.x
author: shaun
status: published
aliases: SD-Card-Testing-from-Linux-under-WLS2.html, Sample-SD-card-mount-WLS2.html
---

## Symptoms

You want to check whether an SD card really has the capacity it claims (fakes report a large
size but silently lose data past the real capacity), and the only Linux you have is WSL2.

## Fix

Mount the card's drive letter (here `E:`) with `drvfs`, then run
[f3](https://github.com/AltraMayor/f3):

```bash
sudo apt install f3
sudo mkdir -p /mnt/e
sudo mount -t drvfs E: /mnt/e
f3write /mnt/e
f3read  /mnt/e
```

`f3write` fills the free space with test files; `f3read` reads them back and reports corrupt
or overwritten sectors. A genuine card shows no losses. See the
[LinuxReviews guide](https://linuxreviews.org/HOWTO_test_SD_cards_and_identify_fake_ones_(mostly_sold_on_ebay))
for more background.

## Cause

Not a bug, just the missing step: WSL2 does not auto-mount removable drives, so you have to
mount the letter yourself with `-t drvfs`.