Under UK MoD (our DoD) rules you have to crush, pulverise (as in reduce to powder) then incinerate media used for secret information.

The overwriting is OK for non-sensitive material.

Questions to ask:

1. How valuable are the data to a third party.
2. Are they aware of existence and location.
3. How current are the data (last week's football results are public knowledge.........with next week's you could make some serious money?)

If you are worried about hi tech recovery.........the Marianas Trench is a good repository........not sure if I could trust what lives at the bottom though?