You probably know that dragging a file to the Trash icon doesn’t actually delete the file; to do that, you need to empty the trash by:
- right-clicking the Trash icon and choosing Empty Trash
- choosing Finder > Empty Trash, or
- pressing Shift+Command+Delete while in the Finder.
However, you may not know that emptying your trash doesn’t delete the file either! Like most operating systems, Mac OS merely deletes the pointers to the file’s data on your hard drive; the data remains, and is recoverable if you know what you’re doing (for example, if you’re a hacker looking for credit card numbers!).
Really emptying the trash
Luckily, Mac OS provides a way to not only delete the pointers to the files in the trash, but also to overwrite the files themselves with garbage data. To do this, switch to the Finder and choose Secure Empty Trash from the Finder menu. It may take a while to overwrite all the data. Once done, your deleted files really are history.
Apple has a help page about deleting files and using Secure Empty Trash.
Wiping the whole disk
If you’re selling your Mac and you’re extra paranoid, you may want to wipe all the data on your disk beforehand. You can do this using Disk Utility’s secure erase options. More information in this Apple document. The truly hardcore will want to choose the 35-Pass Erase option, which takes forever, but makes the data nearly impossible to recover using current technologies.
Enjoy your privacy!