Will re-installing Windows reduce the storage space on my hard disk?

Q  I’m a bit confused over the available storage space on my hard disk. I understand there is software that can delete photos from a CD-R disc, but the space they occupied remains unrecoverable.

Is the same true of hard disks? If I re-install Windows, for example, will the space taken up by the old Windows installation remain forever unavailable?
Edgar Mills

A  This can be a confusing issue. It is helpful to understand that CDs and hard disks use different technologies to read and write information.

To write (record) information to a CD, a laser burns microscopic pits onto a metal substrate – the shiny bit below the clear plastic surface. The pits represent data. This is true also of DVDs and Blu-rays: collectively, they’re known as ‘optical media’ (because the read/write laser is focused using optics).

Hard disks, on the other hand, use a magnetic head to change the magnetic state of the disk’s surface, again creating tiny patterns representing computer data.

Now, to answer your question, there is no practical limit on the amount of times a hard disk’s magnetic surface can be written to, overwritten or erased. In other words, assuming the device doesn’t develop a fault, data stored on a hard disk can always be erased to recover space.

If you re-install Windows, while the operating system may offer the option of retaining old files and folders, there is no reason the current occupied space cannot be overwritten – it’s up to you.

However, the same isn’t always true of optical media. While modern CD- and DVD-recorder drives are able to read and write (or burn) information to a wide range of disc types (the media), not all optical media is equal.

In particular, the surface of the CD-R (or CD-Recordable) disc type you mention can be written to one time only: once the data pits are burned onto the substrate, they cannot be erased or overwritten.

However, other optical disc types, such as CD-RWs, have different substrates that allow stored data to be erased or overwritten – the ‘RW’ part stands for ‘rewritable’.

To finish, though, we think some of your confusion may stem from a broadly outmoded CD-recording technique called ‘multisession’. With multisession recording, it is possible to write data to a CD-R (or DVD-R for that matter) in several sittings.

This might sound like we are contradicting our earlier statement that the surface of CD-Rs can be written to only once – but multisession recording is an incremental process. If a CD-R has 700MB of storage space, for example, it would be possible to burn 100MB of data to it today and a further 600MB at a later date.

To be clear, though, it is not possible to erase the first 100MB: whatever data is there, will stay there (and be accessible) until the disc is destroyed – so you cannot delete old photos from a CD-R.