With a magnetic hard disk, the writes from shred will overwrite the sectors which contain the data. There is a risk though of copies of the data being left on the platter in the event of bad sector remapping though.
With a SSD, the writes from shred will be to new flash segments. With flash memory, erases and writes are separate operations. A SSD keeps a number of pre-erased segments around which are used to hold new writes whilst the old segments are erased in the background.
At a higher level, depending on what applications manipulate the data, you may also have to watch out for deleted temporary copies of your file. A FS defiling tool (or at a minimum dd if=/dev/zero of=crud; shred -u crud) can help remove those traces.
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