Check if sd-cards and usb devices are fake.
Trivial program that tests sd-cards and usb sticks by first completely filling a device with random data and then attempting to read back that data.
This is ment to identify fake devices that report an incorrect size.
A faster test would probably be to write to the end of the device and see that the data can be retrieved.
./flashtest [--print-range] <device>
By default the test exits as soon as a read fails. With --print-range the test continues and tries to read the whole device, printing the failing ranges.