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I have bunch of old hard disks, USB sticks and memory cards I want to sell / give away. However, there's private content that must be cleared before that.
What is a good, preferably free, software for OS X to wipe the contents so that restoring anything is not reasonably possible?
The built-in Disk Utility does everything you want.
To wipe a drive:
Open Disk Utility (It's in ~/Applications/Utilities)
Find the drive you want to be wiped in the sidebar, and select the root drive (not the partition):
Go to the erase tab:
(optional) If you're paranoid about your data, there's a button Security Options. Clicking it brings up a dialog with a slider on it. At the highest security level, the erase mode "meets the US Department of Defense's 5220-22 M standard for erasing magnetic media". It overwrites your files seven times.
Choose a name for your drive and click the erase button.
Click erase on the resulting dialog. Disk Utility goes through its process (which may be rather long if there's a lot of space on the drive).
Enjoy your brand newly formatted drive!
For paranoid shredding, install GNU coreutils and use its shred program.
Disclaimer: any solid-state drives, USB sticks, and memory cards that use wear levelling can't be guaranteed to be fully shredded. This applies to pretty much any disk-wiping software unless they can get beyond the wear-levelling layer.
