Thanks muungwana, but I had not missed that .....
My understanding of the two is that,let say you have a 20MB file,the traditional way of erasing it is overwriting the file with random data and then deleting it,with wear leveling technology,the overwriting part of the operation may happen at a different location on disk,leaving the original file it tack,the file will be copied somewhere else and the overwriting will happen over the "new" file that was copied over before the writing commence and end result is that the original file is still around and a new file has overwritten data and then get deleted.
This brings me back to my question about using a file to fill all available space, writing zeros to it, and then deleting it.
When a file is moved due to wear levelling ... well copied really .... the original is still there but not referenced any more, which *could* be a privacy/security risk at some later time.
So .... the problem appears to be that the time from the original being copied to another location and when that original location is flagged as available to be reused, is unknown and may be considerable?
If it is not flagged as available, then writing to free space will not overwrite it?
Is this what I was missing? --- the time between the two actions?
Thanks for the further explanation