I maked the second test. results :
Warning: Only 12065 of 255999 MByte tested.
The media is likely to be defective.
11.2 GByte OK (23692800 sectors)
496.2 MByte DATA LOST (1016320 sectors)
Details:148 MByte overwritten (303104 sectors)
0 KByte slightly changed (< 8 bit/sector, 0 sectors)
348.2 MByte corrupted (713216 sectors)
148 MByte aliased memory (303104 sectors)
First error at offset: 0x000000000ac04000
H2testw version 1.3
Reading speed: 14.4 MByte/s
you can get the sandisk cruzer glide 128gb or 256gb flash drive i have use the 128gb on my wii u before and it work just fine no errors but i would recomend just getting portable hard drive the one im using right now is a Seagate Expansion 1TB Portable External Hard Drive you will NEED to get a y cable for this almost any hard drives need it so it can get enough power to turn on and be use on wii u
Y cables are for hard drives. They're required to give enough power to a hard drive. Flash drives don't need them. Your flash drive is fake and even though it was sold to you as a 256GB, it only has 12GB on it. As others have said, get a refund. Nothing will fix this, because nothing's wrong with it. It's fake.
Read this article and you will better understand how your flash drive is fake and why it appears legit at first. They report inflated capacities via the microcontroller to your OS. Testing them with utilities is useless unless you do a write test beyond its true capacity. Read tests uncover nothing because flash drives don't report read errors when accessing non existent memory. Even writing to them is deceptive if you don't verify the data afterwards since written data with nowhere to go will either overwrite existing data, or end up in the mysterious "bit bucket" or null device.