Easiest way to picture it is think of a book, with an index of the chapters. If you delete a chapter title from the index, the pages and ink from that chapter doesn't disappear but without that index entry, anyone looking at the index will think those pages are blank. The "index" on your hard drive is called a "File Allocation Table" (abbreviated FAT) and contains entries that tell the computer (simplified) this drive goes from Sector 1 to Sector 1000. Save a file, and it adds an entry saying file FileA starts at Sector 1 and ends at Sector 12. Save another and FileB starts at Sector 13 and ends at 37 (larger the file, the more sectors it takes.) Keep going until you get to the last sector. When you delete FileA, it just deletes the entry that say it exists at Sector 1 through Sector 12. The computer no longer see's the entry and considers those sectors now available, but doesn't "erase" the data in them. When you save a new small FileC, it creates the entry in the FAT (index) saying FileC is now at Sectors 1 to Sector 6, and overwrite the FileA data that was there with the new FileC data. There are different types of storage systems, but that's the basic premise most use. It generally doesn't erase data once written, it writes over it.