There is no such thing as a 512GB Sandisk USB flash drive!
The flash drive offered here by USA registered ebay seller bgwwatters are fake fake fakity fake! Sandisk do not make 512GB flash drives and nor does anyone else. Any flash drive you see in this packaging will be fake capacity. Steer well clear of them.
As usual we strongly advise ebayers to test all flash memory with the free program h2testw.
Report in to SOSFakeFlash if your testing confirms you have a false capacity device.