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I would like to sell my old laptop but after reformatting all. I'm still worried that the new user is able to get back old the old data's. How to prevent this, without damaging the laptop.

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3 Answers3

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To securely erase a hard disk will require need to overwrite the data blocks repeatedly so that remnants of previously stored files are much harder to recover. Secure erase software, in a nutshell, does the following:

Think of a puzzle, if you have all the bits then (at some point) the puzzle can be pieced together in its entirety. If you remove a number of the pieces (full erase 1st pass) then you will still be able to work out what the puzzle picture is even with the few bits missing. If you remove yet more of the pieces (full erase 2nd pass) the picture will become more degraded but could still be identified. The number of passes (or erase and randomly written data) means the recoverable "picture" gets less and less easy to identify - until such point that the picture is unreadable.

Moving on from this... your steps:

  1. Make sure you have the original OS recovery disks (usually on DVD). This is the most important step as it'll allow you to recover your OS once the secure erase of the hard disk is complete!
  2. Download a secure format/erase tool such as DBAN (a bootable CD/.iso) and write it to a CD-R.
  3. Boot to this disk where you can select your HDD to erase. Please make sure you disconnect/eject all other disks before using DBAN as once it's started it's pretty destructive! The CD with DBAN on cannot be erased so don't worry about this disk. The erase process can take many hours (even days or weeks depending on the number of passes and size of hard disk).
  4. Once the secure erase is complete you can reboot your computer and boot to your OS recovery disk(s) to reinstall your OS as if your device had just left the factory.
Kinnectus
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Do not use quick format. That will only erase the file system, which holds information about files: names, date, size and location on the disk. When you use a normal format, all data is erased. You can use tools to securely erease your harddisk multiple times, overwriting with random data. It appears to be possible to read out data from a disk that is overwritten completely, and overwriting it multiple times should prevent this.

Google: how to securly erase your harddisk

Last month I formatted a used harddisk. It had Ubuntu installed with filesystem Ext4. I repartitioned to different partitions sizes, then reformatted to NTFS using quick format, then installed Windows 7. When finishing that install, my colleague came in and told me that the harddisk could have documents on it that he needed. I took the harddisk out, attached it to another Ubuntu desktop, installed the tool Photorec and scanned the drive. I recovered thousands of files, most Ubuntu system files, and hundreds of photos, documents, videos, name it.

To me this was a big eye opener.

If you want to prevent this from happening in the future, encrypt your harddisk. Linux and OSX provide this option by default. Windows does this only for the much more expensive licenses, not the Home editions.

user
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SPRBRN
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I can't comment due to reputation, but would like to expand on Big Chris' answer.

DBAN is a .iso file which you download then burn to a CD. You then restart the PC and boot from the disc, choosing which partition of hard drive you would like to wipe. It will delete everything, then overwrite it with nonsense data and repeat a few times. The reason this is a better option is that if you just delete a file, all you are actually doing is marking that space as 'ok to overwrite' You don't actually get rid of anything - hence it is recoverable.

After using DBAN, the only thing that is recoverable is nonsense.

Joe
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