Q: How do I use the "Zero Out Data" option?

That's a dangerous option if your hard drive directory is damaged, which can often be the case if it is full.  You really have at least two larger external hard drives in terms of GB/TB capacity from some place like http://www.macsales.com/ to keep archives of infrequently used data in their own partition, and a clone of your primary data in on a separate partition.

To learn more about partitioning, see this tip:

http://discussions.apple.com/docs/DOC-3003

To learn more about cloning, see my FAQ*:

http://www.macmaps.com/backup.html

Keep your internal hard drive at least 15% free.  Always have at least two separate copies of all data at all times.

* Links to my pages may give me compensation

Zeroing data is not going to get you any additional free space.  You need to look at the files you have on your system and decide what you want to keep and what you want to move to other storage, and what you want to throw away.  Or depending on the type of Mac, consider if you want to just replace the current storage device with a larger capacity unit (assuming that is possible on your Mac).

OmniDiskSweeper (free download)

<http://www.omnigroup.com/more>

When using OmniDiskSweeper, or any utility that shows all your files...  See the following article if you want to run it as root

<http://www.macobserver.com/tmo/article/how_to_recover_missing_hard_drive_space>

Boiler Plate Warnings:

If you have a recurring, running out of disk space, problem, then OmniDiskSweeper may help identify where the space is going.  Posting the suspected locations and files will help the forum help you to figure it out.  Remember, we cannot see into your disk, you have to give us information to work with.

DO NOT delete files in your Home Folder -> Library tree as there are things like your iPhone backups, your email messages, your application preferences, etc…  If you think you have found something in your Home Folder -> Library that can be deleted, you should ask first.

DO NOT delete files outside your home folder, as you may end up deleting something essential to Mac OS X, and turn your Mac into an expensive “Door Stop”.

I will point out that you will find some very large files in private -> var -> vm (these are the Mac OS X virtual memory paging files (swapfiles) and where Mac OS X stores the copy of RAM when your Mac is put to sleep).  The swapfile(s) get deleted on reboot, and the sleep image is just going to be created again when you put your Mac to sleep.

If you think you have found something to delete outside your home folder, it would be best to ask first before deleting.  There are many examples of people deleting files outside their home folder, or renaming files, or changing the ownership or file permissions, and then their Mac stops running.  Do not be one of those people.  Ask first.