Just to add one wrinkle here...
Once you write a block to an
SSD, it may be IMPOSSIBLE to erase it, since flash parts only support a limited number of write cycles. So when you attempt to overwite that block, it might just fail to change it. Now what will really happen is that the wear leveling logic may remove the block from the write list and replace it with another unused block (some have spares) or something else (disk gets smaller). Not sure. But there may well be ways to get the data on those permanently written blocks back if somebody really wants to see them.
Course you can just cut the little
SSD drives we use in half with a pair of wire cutters...