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How do I erase my hard drive?

Cleaning utensils

There are many reasons why you might want to clean your hard drive. Simple house-keeping is the most obvious, because a clean hard drive means better performance all round. Personal security also counts when you decide to get rid of your PC – either for sale or scrap. There are also times when you’ve accidentally downloaded something you wish you hadn’t, and need to get rid of it.

Jane Fae | 9th September 2010

A different level of cleaning may be appropriate for different jobs. But first, you need to have some idea of how your PC usually stores data. The following advice applies to most common operating systems.

Information is stored in files, and files are made up of fixed-length blocks of data. A large document may contain several hundred such blocks. As you create new documents, your PC fills up its hard drive, grabbing blocks as it goes. When it is new, a file will be made up of blocks sitting neatly next to one another.

As you use your PC, and start deleting old files and creating new ones, the route between the blocks that make up your files will start to resemble a tangled mess of string.

’Defrag’ often

That’s why the quickest and simplest way to tidy up your hard drive – and often to improve performance – is to ’defrag’ your PC. The defragmentation process (likely to be found somewhere in your system tools) untangles the mess. Any blocks that can be rearranged are shuffled around, until documents are (as far as possible) made up of continuous data clusters once more.

There is no magic to this. Just remember to turn off other programs before starting – and allow several hours for the process to run.

The second important consequence of data storage is that your PC keeps tabs on which blocks make up a given document. This is done by something called a file allocation table (FAT). When you delete a file, it isn’t actually deleted. Instead, the FAT is amended so that the blocks that make up a particular document are marked as available for use – but their content remains unchanged.


Delete in Windows, and the document goes first to the recycle bin, from which it can be restored at the click of a button. Remove it from the recycle bin – or even reformat your hard drive (a process not for the faint-hearted) - and the data is still there, and capable of being recovered by advanced PC tools.

…then really delete

If you really, really want to get rid of files or confidential data, you need to invest in a specialised security tool. Look for “complete deletion” or “secure deletion”. Such packages go through your hard drive and systematically overwrite every single unallocated byte of data with a zero – not once, but a dozen or more times.

After that, unbelievably, the data is still not completely gone, but anything deleted by these means is unlikely to be recoverable, unless it’s by MI5 or the CIA!

Jane Fae

Jane Fae

Jane is a consultant on database marketing and crm, as well as a nationally known writer on issues of political and sexual liberty. She also writes for the Register, one of the world's biggest online tech publications, about current affairs, policing and the law as it impinges on technology users.