If you want to erase an SD card completely, there's an official tool from the SD Association here:
SD Formatter 3.1 for SD/SDHC/SDXC
This software formats all SD memory cards, SDHC memory cards and SDXC memory cards. SD Formatter provides quick and easy access to the full capabilities of your SD, SDHC and SDXC memory cards.
The SD Formatter was created specifically for memory cards using the SD/SDHC/SDXC standards. It is strongly recommended to use the SD Formatter instead of formatting utilities provided with operating systems that format various types of storage media. Using generic formatting utilities may result in less than optimal performance for your memory cards.
Shame about the lack of a Linux version (that I could find)
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
Contrary to popular belief, humorous signatures are allowed. Here's an example...
“I think it’s wrong that only one company makes the game Monopoly.” – Steven Wright