Here is the definative answer to this very popular security question:
A hard drive stores "written" coded information! Even a picture/video is written as code in 1's and 0's. The Computer, re-assembles them.
The 1's and 0's are stored magnetically, on an optical disk, you can "see" magnetic wave variations on a microscope, as valleys and hills, on the disk. At this level of magnification, the disk looks like the world!
No human can make sense of the indentations, at machine code, 1's and 0's level. They use forensic programs, to painstakingly re-assemble the data.
The data is not stored in one place, sequentailly, like an lp record, but where ever it fits, as the disk spins, logged by an index.
Imagine an lp record. half an jpeg picture is stored half way throgh song one, the rest at the end of the last song.
When you delete an item, the computer simply "forgets" where to find it. THIS looses the "adress mark", an important pice of written data that tells it WHERE the parts of the image(for eg.) are. It is still there.
Loosing the address of data, alows the system to "see" this tiny part of the disk as FREE, and write new data over it, making new magnetic waves.
You can "see" the old waves, upto 3 re-writes, BUT only relative to the cost, time, experience of the forensics.
A re-writen disk, 1 to 2 times, cost about 5000 pounds to find the origional data, and "mod" or "fbi" equivelent sub contractors, to carry it out, with knowladge of the sort of thing thery looking for.
If you inagine those metal pin frames you press your hand into, to make a 3d shape of your hand, if you then use your foot, being bigger, youll get a print of your foot, BUT some of the pins will only have gone more in, and other, up pins, gone in, so the origional hand is still really there, if you konw it was a hand, and analise the movements of the pins, youll "see" it.
What you want is something to push all those pins, back to the top. Because of magnetism, this is NOT possible with a hard drive. You can only re-write it. Once, a reeboot, is sufficient for Joe average and snooping, (readily available "forensic" programs), Twice for police (say, MISTAKENLY!) innappropriate material, and 3x plus for terrorist (serious) investigation, with no hope if the authorities budget feel its worth it. BUT! I mean, a reboot, and COMPLETE over-rite! Dont confuse a re-format, with a clean system, those indentations have to be written to again, by the operating system, and spare disk space, in temp files, programs, etc. Thats the overview, now the a-b easy guide:
I WANT A CLEAN HARD DRIVE HOW?
1) If the information is really sensitive, (youve mistakenly loaded child porn). Remove hard drive, take out disk. Bonfire. (new disks are cheap, think of the social stigma, even for a caution!) Do NOT dispose of the disk as a whole. With the disk out, to put it in another casing, and read it, lining up the optical pin arm, is reasonbal impossible! SO DISMANTLE the disk, separate the "platter"!
2) if the information is financially/personally sensitive, really so, see above, or below if not so worried.
3) Re-boot with windows, and driver disks (if you are confident) and surf loads of junk, videos, pictures, download space filling crap (the above), increase the temporary cache to max, "save dvd's to disk, fill it up! Re-boot again. Sell/give away (if thats why your clearing the disk)
4) Re-boot, with a program like "D-Ban", a free progam, Ive only used with 9x (win98/me) which should overwrite hard drive with 1's and 0's (the simplest form, more than enogh for snoops) by three times, which it does automatically. CAREFUL, I dont know the compatibility with xp, and even then, feedback on google has said its a virus, because they didnt properly understand, IT WIPES ANY FOUND STORAGE MEDIA in sequence, once started, (a second hd disk for eg.) AS ITS PROGRAMED TO DO SO. You definatly need a boot disk for this one, forget system restore!, ensure the BIOS are set up to read from CD if u dont have floppy boot, etc, as it is not "user friendly! It says this on the instructions. If you have suitable boot media, can check a bios to make SURE you can boot again from your chosen media, Dont have data on a second hd (you dont want clearin) then its a good "honest" free ware program. At the level it runs, it cant introduce viruses, but with rootkits, ? check the validity of the down load site, mine was on a "geeks" forum with no advertising!
5) if not too fussed, re-boot with windows cd, and put on any software your including for sale/ give away...most people will soon have so much crap on it themselves!(be nice, do the windows updates before sale, put zone alarm on it etc)
TO SUMARISE:
Because its ALL written data, sensetive stuff, particularlly origionally written data such as finance details, word documents, etc, are stored as a billionth of a millimeter of disk space. (computers love words, 1's and 0's in code, and could put shakespear in the same volume as one jpeg picture) Be very careful about small pices of sensitve data, RELATIVE to the "snoops" resources.
Overwrite to sturation, when re formatting
A reboot does NOT clear data
If in doubt, remove PLATTER (80% of making it secure), more than particular method of destruction, of "intact" drive! And destroy.
Use an overwrite program, if not loading disk with Junk, But want to give it away or sell it Reasonably securley.
Just re-foramt and use, if you just want a clean start, you'll soon over write.
IF you have made mistakes? re innapropriate material, you must overwrite, after a reeboot, which will soon happen, and even if forensic exams shows Thumbnails, we have all be directed to inappropriate pages and the authorities know this. Any follow ups, repeated clicking on more depth from suck links, your isp logs will be investigated, coupled with evidence from your hard drive, and you should distroy the disk as above, and shame on you for your morbid curiosity, NOT saying this is you, but at least the info is there, candidly, for anyone who HAS been "mislead" and thankfully are comming to reality!