To be sure that your data is erased completely from the drive you should:
1) Install the drive as a slave, so you can boot from another disk.
2) Use a program to wipe each and every sector of the disk, preferably with a known pass pattern. Jetico have a utility called BCWipe which will wipe the disk with known data patterns and can be set to your own perferences. US DoD spec is 7 passes of 35 CA 97 68 AC 53 Random, Peter Guttman is 35 passes.
There is little need to have 7-35 wiping passes, 3-4 will suffice, only specailist agencies would be able to recover from more than a 4 pass wipe, and depending on the patterns and many other factors it would prove time consuming and costly for them to conclude your getting rid of porn. Four passes will almost certainly have realigned the entire magnetic surface making recovery almost impossible. Remember wiping is about aligning and realigning each part of the magnetic surface until the alignment is quasi random.
A format, or removal of partitions is not sufficent, free software can recover all the data from a formatted drive, even a full uncondidtional format.
As for gov/mil sensitive data, the drives are usually melted in a furnace, nothing more elaborate than that, as any tiny fragments could yeild data.