Clearing out unused disk space
There is a windows command you can use to overwrite empty space on a drive. This is useful because deleting a file actually just disconnects it from all folders, but does not destroy the data.
cipher /w:driveletter: overwrites the unused space on the specified drive with all zeros and then all ones and then random numbers. For my purposes, this is all I need to feel pretty sure my old deleted files will not be recoverable.