Yes it’s true that deleting a file or emptying the recycle bin doesn’t really delete the file. It’s still on your computer. Think about this: Why does it take longer to copy a 1GB file on to your computer, but it takes only less than a second to delete it? The answer is very simple: When you hit delete, it doesn’t delete the file. It only remove it from the interface, where you can’t look it up, but it’s still in the harddrive.
You will need a special software to actually delete it, and it does this by overwriting the file with random numbers. If it takes you 3 minutes to copy the 1GB file, then it’ll take 3 minutes to clear it. However, special softwares can still RECOVER it! So you will need to increase the repetition (number of times to overwrite that same file). So if you want to do it 4 times, then it’ll take 12 minutes. Doesn’t sound much for a 1GB file... but if your Harddrive is 500GB, and you want to make sure that it’s wiped clean, then it’ll take much longer. Don’t worry though, the software will take care of all that for you.
Usually if you’re on macOS, you should have encrypted your whole drive from the start, so that everything that gets written to it is automatically encrypted by default.
If you’re on PC, I think a better option is just take out the harddrive and put in a brand new one and sell the PC with that.
If all you viewed were just regular P, and nothing questionable then nothing to worry, just use a special software to do a quick erase. But if you got stuff on there that might be bad for you down the road, I would take out the harddrive for peace of mind.
EDIT: Oh, its a Laptop you’re selling. In that case, I suggest you look for a special software designed for erasing data.
Just a piece of advice: People might say they’ll clear up everything for you once you sell it to them, but you don’t know that for sure. If you got stuff on there that might cause misunderstanding and ruin your reputation down the road, then you need to be wiser than just taking them at their words.
