When you reformat magnetic media 'securely' you write random data across the entire surface, because iron remembers previous magnetisation, leaving slight variations in the magnetisation of the new data, and sufficiently sensitive equipment can decide what was on the medium before the current write.
Indeed, I remember reading in the 1980s that a lab had managed to track more than seven previous writes. I was working for a defence contractor, where there was a storage room full of dismountable disks that had had classified information stored on them, and they were classified 'Top Secret' for eternity.
My question is, do USB sticks and SD cards also have a memory of previous contents? Am I wasting time doing more than one write of the volume?