USB mass storage device tester is a small application that:
– looks for any USB mass storage devices connected to the Wii
– tries to display a file listing of the root folder
– creates a file called testtest.txt with the content ‘test’ there.

WARNING:

FOR FAT (12, 16 AND 32) DEVICES ONLY
USING THIS ON ANY DEVICE THAT HAS A NON-FAT FILESYSTEM ON IT WILL RESULT IN DATA CORRUPTION
CREATE A BACKUP BEFORE USING THIS SOFTWARE

http://wiibrew.org/index.php?title=Homebrew_apps/USB_mass_storage_device_tester