A good way to check whether an SD card is genuinely the capacity printed on the label is to perform a full write-and-read verification test. Fake cards often report a larger size (for example 128 GB) but actually contain much less memory (for example 16 GB). Once the real capacity is exceeded, older data gets overwritten.
The most widely used tools are:
Windows
H2testw (free)
Download and run H2testw.
Insert the SD card.
Select the card and choose “Write + Verify”.
The program fills the card with test files and then reads them back.
If the card is genuine, it will report no errors and the expected capacity.
If fake, it will report the true usable size and data corruption.
Linux
F3 (Fight Flash Fraud)
Install:
sudo apt install f3
Write test files:
f3write /media/your_sdcard
Verify:
f3read /media/your_sdcard
It reports whether the full capacity is usable.
macOS
F3 can also be installed using Homebrew:
brew install f3
Then use the same f3write and f3read commands.
Things to remember
Back up any existing data first. These tests fill the entire card and may erase or overwrite data.
The test can take several hours on large cards.
Buy SD cards from reputable retailers to reduce the risk of counterfeits.
For a PicoMuon, it is worth running one of these tests before relying on the card for long unattended data collection. Even cards from well-known brands occasionally turn out to be counterfeit or defective.