You've probably bricked your flash drive by attempting to load an .iso with it using a USB loader.
You should know that it is widely, highly discouraged to USB-load .isos from a flash drive, as opposed to a HDD/SSD. Flash drives simply aren't meant to endure the extremely frantic read-and-write operations that disc-based gameplay entails. They're meant to be used by loading one, or very few, files at a time, but what your Wii is doing is decoding the .iso, grabbing files from it to put into the RAM and evict from the RAM to grab other new files from the .iso, very quickly. This creates a whole bunch of wear-and-tear on your flash drive REALLY DAMN FAST.
If you want to load Genesis/SNES/NES/GB(C)/A ROMs from your flash drive, that's fine since it loads the ROM only once into RAM and is read from there. ISOs are read constantly from your flash drive every time something is loaded in-game.
Solution: Get a USB HDD/SSD, and save yourself the trouble of going through flash drives like popcorn.