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What does Format Do?

I've been formatting all matter of media for years, but I'm unclear on what exactly formatting even is. I know that it prepares the disk to be used and I know that you can format disks in different ways for different purposes and operating systems, but beyond knowing I should use format style ABC for operating system XYZ, I have absolutely no idea what the formatting process actually is.

What does formatting a hard drive actually do to the hard drive? How is a raw and unformatted disk right out of the box different from a hard drive that I've formatted for use? And for that matter, what's the difference between quick format tools that complete the process in seconds versus more in-depth formatting that takes hours on a large disk?

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  • The physical or low-level formatting of a hard drive is the process of writing an address mark, the ID record, a (blank) data record and all necessary gaps for each sector on every track of the HDD. After the format, when a sector is "written", only the data record (and its leading & trailing gaps) of the sector is rewritten. The address mark and ID record are never rewritten after the format. More details here: superuser.com/questions/432318/what-are-disk-sectors-for/…
    – sawdust
    Oct 11, 2012 at 19:23
  • With ATA (IDE) drives and later, the only place a hard rive is ever low-level formatted is at the factory. The only place it can ever be done is at the factory. There's nothing you can do at the drive's ATA or SATA connector to "low level format" it.
    – Jamie Hanrahan
    Oct 16, 2014 at 8:26

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Formatting is necessary to actually store data on a disk. The term is sometimes used interchangably for both low and high level formatting.

Low level formatting is the actual setup of the media with respect to the controller, and hasn't been something that you've had to do manually for over a decade now. Drives come with this already done and most people never need to worry about this.

What you're thinking of is high level formatting. This is the process by which you actually partition and set up the file system of your choice on the drive - it installs the boot sector, file table structure, or whatever the filesystem needs in order to be recognized. Once the file system is in place, software that can read it can utilize it. (Quick format versus long format is purely a question of how the formatting works - quick ones usually just set up the structure and ignore anything else that might be on the disk, while long ones will generally zero out anything that might have been left over from a previous partition).

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  • This stuff is a lot more complicated than this, so I've simplified a bit to keep with the tone of the question as asked.
    – Shinrai
    Oct 11, 2012 at 18:02
  • You don't need to format your drive to store data. You can perfectly use it and write data on it without formatting and without a file system. I would say you need to format to be able to easily manage the data on a disk, which is considerably more difficult if you don't use a file system.
    – Marco
    Oct 11, 2012 at 18:05
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    @Marco - I did mention I simplified a little bit. Practically speaking, a drive without a file system isn't going to be useful to the man on the street.
    – Shinrai
    Oct 11, 2012 at 18:10
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    @Marco - "You don't need to format your drive to store data" - That's only because modern HDDs are already low-level formatted from the factory for 512 (or 4K) bytes per sector. If you're old enough to remember, HDDs used to be sized by unformatted capacity and inflated with power of "10" rather than "2" "megabyte" values. So the actual usable data capacity was like only 70% of what you thought you bought.
    – sawdust
    Oct 11, 2012 at 18:52
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    @sawdust - I don't know when the last time you bought a hard drive was, but last I checked they're still sold based on powers of 10 rather than powers of 2. (Usually if you read closely they'll at least admit that these days)
    – Shinrai
    Oct 11, 2012 at 19:44

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