What It Means and What You Can Do
Formatting a hard drive is not a light task to take on. When you do it, you erase anything and everything that is saved to the disk, with no chance to get it back without the use of some expensive software. If you format a hard drive, whether you format a Seagate disk or a Toshiba disk, you lose everything on it as soon as you write new information to the drive. Knowing that fact, make sure you have any files you want to keep saved to either another hard drive, a flash drive, or an external hard drive before going through with a full format. If you don't have a secondary place to save your files, I would strongly recommend you wait to format, but if you're still dead set on it, feel free to read on.
Step 1) Connecting Your Hard Drive
There are two types of hard drives you can format: The start up disk, and a secondary drive. The start up disk is the hard drive that loads immediately when you push the power button on your computer. It's where all of the primary files on your computer are stored, and it's much harder to erase than a secondary hard drive. Not impossible, but hard.
A secondary hard drive is easier to format and where you store secondary files. It doesn't hold any necessary files for starting up your computer so there is much less to worry about. Essentially, you could unplug your secondary hard drive at any time and not suffer any consequence in the long run. This first step only really applies if you're formatting this type of drive. All you have to do is make sure it's connected via USB or FireWire or the internal hard drive ports, and then you're good to go.
Step 2) Opening Disk Utility
Note: This step applies only when formatting a secondary hard drive.
After you've connected the hard drive, you'll want to open up our good friend, Disk Utility This utility is found either on the dock, in the "Utilities" folder, or within Spotlight on your Mac. Disk Utility is a program that handles all the repairs and formats of various types of storage devices, including hard drives but expanding outward to flash drives and other external devices. What you should see will look something like the image on the right, and if it varies you may want to check to make sure that your program is up to date and that you opened the correct program.
Step 3) Select Your Hard Drive
When you open Disk Utility, on the left hand side of the window will be a list (in the screenshot above, the list contains my hard drive and my DVD drive) of all the storage devices and disk drives currently connected to your computer. Make sure you know which one is the hard drive you want to format. If you are unsure which one is the correct one, shut down and disconnect the drive, read any labels present on it, write down any names, and then retry steps two and three. Look for the matching insignias, logos, brand names, and labels (such as "Seagate" or "Toshiba") in Disk Utility that are also present on the physical hard drive. Another thing to look for is the hard drive size. If you know your hard drive is 500GB and there is only one 500GB hard drive listed in Disk Utility, then you know it's the right one.
After you highlight the correct hard drive, the right side of Disk Utility will change and you will have a whole slew of new options available to you.
Step 4) Open the Erase Tab
Once you've got your hard drive selected, you'll see a few things on the right side, including tabs labeled "First Aid", "Erase", "RAID", and "Store". The only one you need to be concerned about right now is the one called "Erase", for this is the one that will allow us to format our drive properly. Scroll over the tab, click on it, and then take a moment to assess the new options you have. As seen in the screen shot to the left, you will have a choice of how you want to format it, what you want to name it, and then the "Erase" and "Security Options" choices, as well as a button labeled "Erase Free Space." Once again, the only one of your concern is the one titled "Erase".
But what about these naming and formatting options? What should be selected?
Step 5) Select Volume and Naming Options
You should see a drop down list that will have something called "Macintosh OS X Extended (Journaled)"- or something to that effect- displayed in it. Click on this drop down list, and view the various options you have. If you aren't sure which one you want, just leave it at the default. This will allow you to run OS X most efficiently.
Next, feel free to name your hard drive whatever you want, whether that's "My Hard Drive" or "The Coolest Hard Drive to Grace Planet Earth"; it's totally up to you. I won't judge you, I promise.
Step 6) Begin the Format
After all of this is said and done, just click "Erase" and you're well on your way to having everything completely clean. As the progress bar reaches its destination, don't think too much about what you're deleting, because if you forgot to save something, you're pretty much out of luck anyways. You can't stop it. You can't hide from it.
Okay, you can totally hide from it, but that's beside the point.
All images are screenshots taken by the author.