zdnet[.]com/article/whats-the-safest-way-to-permanently-erase-your-laptops-drive-ask-zdnet/
A conventional hard disk reads/writes data from/to one or more spinning platters. The read/write heads float barely above the platter(s) on a cushion of air. Because they're floating, those heads may wiggle side to side a very tiny amount. When a file is deleted all that happens is the index to that file's physical location is deleted -- the data's still there, which is why you have recovery tools. That data goes away if the locations where it's stored are rewritten with new data. Apps like today's GOTD, AweEraser, are built on the premise that if the drive's heads were leaning right when they wrote the original data, but leaning left when they overwrote that data, there might be traces left that could be picked up by an expert. To prevent that it overwrites the data several times, on the idea that at least one of those times the heads will lean in the correct direction to get anything an earlier pass missed.
SSDs are harder to deal with. You have a Big collection of storage blocks which are written to somewhat randomly. At the same time, because you can only write to one of those blocks so many times, they use wear leveling, trying to make sure every storage block is written to the same number of times. And they have spare storage that they'll swap out if the SSD detects a potential problem. So, when you want to make sure something's really gone, you need to reset the drive, and several manufacturers provide software that can do that for the drives that they sell. Problem is, that software reset doesn't always work, so research before you depend on it.
That said, Ed Bott provides a couple of strategies to thwart the casual user running a file recovery app. His first hint is to encrypt the drive, so that anything left over after deletion is gibberish. His 2nd hint is to use a Windows tool that overwrites unused disk space -- space that may contain deleted files but not any that show up in File Explorer. From a Command Prompt running as admin., type Cipher /W:C:\ and press Enter to overwrite all of a drive's free space. Use Cypher /? for a list of options & what they do.
