True SD Card Capacity Test +
About True SD Card Capacity Test +
It's not the most pretty app because the focus has been instead on real-world testing and good algorithms. It is, however, easy enough to use. The only way to know your card is good is to fill it to the advertised capacity. Typically this takes less than an hour but can be longer if the card is particularly slow or enormous. You can leave it running overnight. Piece of mind is worth the wait and there are no shortcuts to it!
You can also test your phone's internal storage. With USB-OTG some phones will let you test thumb drives too, but many phones don't follow the spec properly and don't give access to the OTG drives.
Note: interface is in English only.