What a Simple Hard Drive Format Does

Formatting a hard disk is a process that users perform for a variety of reasons:

They have to format a new hard disk so they can write data to it

They reformat a disk so they can reuse the space on the disk to write other data

They reformat the disk to clear the data on it, thinking that this removes the data

So – what is formatting, and what does it REALLY do?

A new hard disk is basically a blank canvas.  It already has information about its geometry (number of platters, list of sectors, map of known bad sectors, etc.) that can be read by the computers into which it is installed.  What formatting the drive does is create a map that allows the operating system to create blocks or files, and write them to specific locations on the disk.  This table is referred to whenever the files are to be read or modified.  Using an old analogy where a disk can be looked at as millions of mailboxes, formatting software creates an address for each mailbox, and stores the addresses and links between mailboxes for each file.

When a new disk drive is formatted, these addresses are created.  A ‘Quick Format’, available in versions of Windows going back a decade or more, only formats a small portion of the drive.  When more allocation information is needed, additional areas on the disk are created (formatting is extended for these new areas).

Formatting can be done with a drive already installed in a computer, and in this case the drives are typically formatted one at a time.  This is fine for individual users.  An organization that wants to format drives without installing them into a computer, or that may want to format many new drives at once, has a very useful option that enables them to do this.

Aleratec 1:11 Hard Disk Drive Duplicator and Sanitizer

The Aleratec family of Hard Disk Drive Duplicators and Sanitizers includes a Format feature built into its operating menu.  Depending on the model of the Duplicator, as many as 12 hard disk drives can be formatted at one time.

Formatting only creates or modifies the allocation tables that are used to write the files and find them later.  Formatting doesn’t affect any of the data outside these areas.

Although some people may still believe that reformatting a hard disk deletes the data from the disk, they’re wrong.  This is a fact that Peter Norton knew in the 1980s, and that he used to create one of his first programs, Undelete, that took advantage of this fact.  When a drive is reformatted, the allocation table created when the drive was originally formatted is overwritten with a new one.  This reformatting, in effect, changes the map to the data – but not the data itself.

Software utilities are available to scan through the disk, finding files that were ‘lost’ by formatting, and rebuilding a map to those files.  In some cases, the map may not find everything, and in many cases the first letter of a file name may have changed.  With a bit of work, the files ‘lost’ by formatting can oftentimes be completely recovered.

If a drive is accidentally reformatted – leave it alone until you can use a tool to unformat the drive.  Any changes made to the drive, or data written to the drive after an accidental reformat can overwrite existing data, making it more difficult to recover the old data on the drive.

If your goal is to reformat because you want to reuse the drive, and you don’t plan to let the drive out of your control, go ahead, reformat, and write away.  The more new data you write, the higher the likelihood that you’ll overwrite old data, and make it increasingly difficult to recover the old data.  However, if you’re going to dispose or sell the drive, or you just have private information that you need to make sure can’t be recovered, reformatting is not enough.

In the next installment, we’ll explore how to REALLY remove the data from your drive and make it TRULY unreadable.

Mark Brownstein is a technology journalist and technology consultant who specializes in explaining and interpreting new technologies, and clarifying how to integrate these new products into current systems. He has been Editor-In-Chief at computer technology and networking publications, has held significant editorial positions at major technology magazines, and is a frequent contributor to various technology magazines. He has written seven books. He is Microsoft Certified, and spends much of his time testing hardware and software products, running his own networks, and learning the best ways to get computer systems running and to keep them running.

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