It depends on your paranoia level. In the good old days, there were no levels hiding the disk sectors from the computer. Then came automatic sector mapping for bad blocks. Really convenient. But how are you going to wipe those mapped-out blocks? As far as I know, there is no way to do so. But there are probably only a few such blocks and they are probably somewhat defective. We'll ostrich the problem away. Then came flash memory with wear-levelling firmware between the computer and the bulk flash. And the devices are over-provisioned: there is more flash than "simulate" disk space. I don't think you can be sure to have erased it. Maybe I'm wrong. Some disks have a feature where they have a key that encrypts every block. The key persists in the drive. But if you want to wipe the disk, you change this key. Then every block is nonsense until it is rewitten. I haven't got time to look up the name for this feature. It is optional. I think that most enterprise drives have it.