Canon EOS 600D - Manual (Page 46)

3 Formatting the Card
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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.287).
Execute [Format] in the following cases:
About Low-level Formatting
Do low-level formatting if the 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 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 have been completed and you can use the
card as usual.
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 might be smaller
than the capacity indicated on the card.
This device incorporates exFAT technology licensed from Microsoft.