The SD card you purchased may be an SD card that fake the capacity.
This application do writing test to judge whether it is fake.
Note: SD card adopted as internal storage not supported.

It writes data to the SD card until there is no more free space.
And, read the data once written from the SD card and check that the value matches the written data.

When the test is over, the capacity which could confirm the correctness is displayed.
If the displayed capacity is clearly smaller than the capacity of the purchased SD card, it may be counterfeited.

However, if you check against the SD card where data is originally stored, only the remaining capacity will be checked.
In this case, please judge by adding the amount of stored data originally.