Are deleted files overwritten by new files?

I think the correct answer is...Maybe.

The space occupied by a file that was subsequently deleted will indeed be overwritten at some point in time. When is hard to determine, as this is up to the firmware controlling the hard drive.

Of course if you really want to delete a file do a "Secure Empty Trash" - this will obliterate it...

My guess is you are looking to recover a file?

 

They are overwritten when the OS decides to do it.

But the space once occupied by a deleted file basically becomes immediately available for use.

A whole file is unlikely to be overwritten, but a large single deleted file could get corrupted as sections inside the file are overwritten.

Small files could get completely blasted away.

So if you delete a file accidentally, you want to try a recovery app as soon as possible.

Turn off the machine (or stop using it) until you recover the files.

 

I have not had cause to recover a deleted file since I upgraded to MacOS X. However, I did semiregularly when I used MacOS 8/9. At any rate, my experience is that deleted files remain intact for quite some time. I have recovered files which had been deleted months before. Over time, deleted files become so corrupted by overwrites that they are useless. In that sense, Sun Baked is correct. The quicker you recover an accidentally erased file, the better. However, there is no need to panic. If you delete a file and don't have file recovery software, you have time to buy it and you may continue to use your computer in the meantime.