Running FORMAT on a HD will not erase the data, it will only erase the
pointers to that data. So, if you are really concerned about other folks
reading the data on your HD (as you should be if, for example, you plan
to donate or discard that HD), then you should actively erase the data.
For most purposes, apps like Eraser (google the net to find it) are more
than adequate to erase data. If you have extremely sensitive data, then
you may need to physically destroy the HD.