Posted 01 July 2012 - 13:48
If you're really paranoid, or stored Top Secret government data on a hard drive, the drive needs to be physically destroyed. For everyone else there are all sorts of drive scrubbing apps available, McAfee for example includes this in at least some versions of their AV software.
Erasing or securely deleting files means overwriting that part of the hard drive's storage with new data, usually garbage but all ones or zeros, as with a full format works too. This is because data's stored all over the place on the hard drive's platter(s), with a Table of Contents at the front of the drive/partition recording where all the bits & pieces of each file are -- simply deleteing a file only removes its TOC entries. That's why all those utilities to recover lost files work -- you can fit all the pieces of a file back together like some sort of jigsaw puzzle [assuming all those pieces are still there, haven't yet been overwritten, which is why if there's something *Real* important you need to recover you click off the main power switch or pull the plug, running your recovery app(s) from a boot disc/USB stick]. Now, conventional hard drives work by having heads float above the disk platter(s) to read/write data -- since they float there's a very slight amount of wobble. If as a file is written one head happens to wobble to the right, then when it's overwritten that same head happens to wobble left, there could be traces of the original file remaining alongside the newly written data -- that's why erasing/scrubbing a drive usually means overwriting everything multiple times... the more passes you make the more chances the head(s) were on the right, the left, & everywhere in between overwriting data.
You can also erase, scrub, securely delete individual files, &/or all the free space to get rid of stuff without wiping the entire drive. In that case you might want to use an erasing app that goes after the unused portion of cluster blocks... On a conventional hard drive the storage space is divided up into small chunks & data's stored in those -- one way to think of it is if you set out a row of glasses & start pouring a bottle of your favorite beverage -- the last glass is likely to be only partially filled. If one of these blocks or chunks was completely filled, then only partly overwritten, there's old data still there that could potentially be recovered.