How to Reformat an SD Card in OS X
An SD (Secure Digital) card is a small memory storage unit that people typically use to store files such as pictures and videos shot with a cell phone or digital camera, or to add music to a portable MP3 player. An SD card might be formatted by a particular device, or by a computer. If you need to reformat an SD card with a Mac running OS X, you can easily do so with Disk Utility, a free utility that Apple installs on every Mac.
Slide the SD card's slider to the unlocked position if it is not already unlocked. A locked SD card can't be erased or reformatted. See the manufacturer's instructions for details on the location of the SD card's slider.
Insert the SD card into the SD slot on the Mac, or into an SD card reader that is plugged into a USB port on the Mac. The SD card will mount on the desktop.
Click "Applications" in the Dock, then click "Utilities," and then click Disk Utility."
Click on the icon of the SD card in the left side of the Disk Utility window.
Click the "Erase" tab.
Click the "Format" pull-down menu to select a format for the SD card, such as "Mac OS Extended" to use the card with a Mac, or "MS-DOS (FAT) to use with both a Mac and a PC or another device that may require MS-DOS formatted SD cards, such as a digital camera.
Click "Erase." The SD card will now be reformatted according to the setting you chose.
Right-click on the icon of the SD card on the desktop when you have finished formatting it, and then remove it from the slot or the card reader.