Comment by snek_case

IMO the problem here is that even if your flash drive presents a "dumb flash" API to the OS, there can still be caching and other magic that happens underneath. You could still be in a situation where you write a block to the drive, but the drive only writes that to local RAM cache so that it can give you very fast burst write speeds. Then, if you try to read the same block, it could read that block from its local cache. The OS would assume that the block has been successfully written, but if the power goes out, you're still out of luck.