Canon EOS 5D Mark III Instruction Manual (en)

Before You Start
The card is new.
The card was formatted by a different camera or a computer.
The card is full with images or data.
A card-related error is displayed (p.377).
Format the card in the following cases:
About Low-level Formatting
Do low-level formatting if the SD card’s recording or reading speed seems 
slow or if you want to totally erase all data in the card.
Since low-level formatting will erase all recordable sectors in the SD card, 
the formatting will take slightly longer than normal formatting.
You can stop the low-level formatting by selecting [Cancel]. Even in this 
case, normal formatting will be completed and you can use the SD card as 
Cards with 128 GB or lower capacity will be formatted in FAT format. 
Cards with a capacity higher than 128 GB will be formatted in exFAT 
format. If you format a card with a capacity over 128 GB with this camera 
and then insert it into another camera, an error may be displayed and the 
card may become impossible to use. Depending on the personal 
computer’s OS or card reader, it may not recognize a card formatted in 
exFAT format.
When the card is formatted or data is erased, only the file management 
information is changed. The actual data is not completely erased. Be 
aware of this when selling or discarding the card. When discarding the 
card, execute low-level formatting or destroy the card physically to 
prevent the data from being leaked.
Before using a new Eye-Fi card, the software in the card must be 
installed in your computer. Then format the card with the camera.
The card capacity displayed on the card format screen may be smaller 
than the capacity indicated on the card.
This device incorporates exFAT technology licensed from Microsoft.