Canon MARK IV User Manual (en)

Before You Start
About Low-level Formatting
When [g] is selected, low-level formatting is possible. If the writing 
speed to the SD card is slower than usual or if you want to completely 
erase the data in the SD card, checkmark [Low level format] and 
format the card.
Press the <L> button.
In step 3 on the preceding page, 
press the <L> button. 
The [Low level format] option will be 
checkmarked <X>.
With <X> displayed, select [OK] to 
start the low-level formatting.
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.
Since low-level formatting will erase all recordable sectors in the SD 
card, the formatting will take 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 
SD card as usual.