Canon EOS Rebel T1i Instruction Manual page 41

Execute [Format] in the following cases:
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.209).
About Low-level Formatting
Do low-level formatting if the card's recording or reading speed seems
slow.
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.
The card capacity displayed on the card format screen might be smaller
than the capacity indicated on the card.
3 Formatting the Card
41