So how to you reformat it using an Apple Mac?
First you need to load Disk Utility. To do this click on the Applications folder, and chose Utilities.
Within Utilities select Disk Utility.
Now plug the SD card you want to format into a USB SD Card reader and plug this into a USB port on the mac.
You should see this SD Card appear in the left hand window. Take care with the next step, as you could lose valuable data if you format the wrong thing! I normally tell by checking the size of the disk. My SD Card is 8GB, so it should be fairly obvious which one it is.
You need to Right Click on this disk in the left hand column, then select 'Erase', with the left button, from the menu which appears.
You now have the option to change the Format of the SD Card and to rename the card if you require. Leave the Format as MS-DOS (FAT). I usually leave the name as UNTITLED.
Then click 'Erase...'
A warning window will appear asking if you want to erase the disk. This is the point of no return, only click 'Erase' if you are 100% sure this is the right disk.
In several seconds time you will have a fresh formatted SD Card, ready to use.
All that remains is to eject the SD Card. Right click on the UNTITLED disk icon on your desktop, and select 'Eject "UNTITLED"'.