This question gets asked a lot. The answer is that there is no guaranteed way -- interfaces change so that your device may not even plug into anything in the future; motors in drives seize up and stop turning; file formats evolve so that there may not be software to read your data; bitrot erodes the data right out from under your nose... The best solution that anyone has come up with is to keep it spinning on a live computer, and migrate your data with you when you upgrade said computer hardware/software. If you want to put something in the capsule that's great, but make it a symbolic effort. If you
/really/ want to make sure the data is readable in 16 years, then keep a copy on your computer and don't delete it. Storage capacities blossom larger and larger every year, so chances are you won't miss the drive space.