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Canon seems to be a bit shady about what their low-level format function actually does. I know what low-level formatting means when applying it to old HDDs that had a separate disk controller, but I don't think that has anything to do with the function found on modern DSLRs

What is the difference between low-level formatting and regular formatting of memory cards?

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Which specific model? All of my Canon user manuals indicate only the file management information is changed when the card is formatted in-camera. – Michael Clark Feb 3 '14 at 16:18
@MichaelClark It applies to all of my recent cameras like the 70D, EOS M and the 6D. They all have this feature. – Hugo Feb 3 '14 at 16:26
If the 5DIII is the same, then the Low level format option only applies to an SD card. Of course the 6D and 70D don't have a CF card slot, do they? – Michael Clark Feb 3 '14 at 16:39
@MichaelClark No they don't and neither does the EOS M. – Hugo Feb 3 '14 at 16:41
@hugo - You can't do a low-level format on flash memory. You can send it all the instructions to do one but the 'disk' that the camera sees is not a true representation of where things (including writes) are because of wear levelling. – James Snell Feb 4 '14 at 20:55
up vote 5 down vote accepted

Based on the user manuals, it appears the in camera low-level format does indeed overwrite the entire card. I think your confusion is from the two separate notes in the user manuals for newer Canon cameras that use SD cards. The other note about only the file management information being changed applies to a normal (non-low level) format. That notice has been in every Canon DSLR manual I've looked at for several years including those cameras that only have slots for CF cards.

6D Instruction Manual (p.54)
70D Instruction Manual (p.57)
Rebel T5i Instruction Manual (p.49)

Other Canon bodies that use SD cards have the same information in their respective Instruction Manuals.

Based on this specification, SD cards support the ability to overwrite the entire card quickly. CF cards are not capable of this.

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Ok, but what do you mean by overwriting the entire card? It can't modify each single bit since a low-level format only takes a few seconds on my 64 GB SD cards. – Hugo Feb 3 '14 at 16:45
The only way to "erase" flash memory is to write over it. Maybe the cards controllers have the capability to write all zeroes or all ones to the entire card in a short time. Since the data going to each address is identical, perhaps it can be done in parallel rather than sequentially. – Michael Clark Feb 3 '14 at 16:52
Perhaps the controller can do that. I will wait accepting an answer to the question until the difference between the two formatting methods is clear. – Hugo Feb 3 '14 at 16:56
According to this Canon KB article, low-level formatting will "erase all recordable sectors in the SD card". This option only seems to appear for SD cards, not CF cards. Not sure why that would be the case (perhaps SD has a controller interface to perform a fast low-level erase). – drfrogsplat Feb 4 '14 at 2:25
@drfrogsplat That article is taken verbatim from the 5DIII user manual. – Michael Clark Feb 4 '14 at 3:01

I did experiment with my EOS 70D and 32GB SD Card. The normal format just rewrites the system part of the FAT32 partition, leaving data intact. The low level format really sets the whole partition data to zero, interestingly enough in approximately the same time.

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SD cards support the "ERASE" command which can be used to clear a large number of blocks quickly: — see also… – Jukka Suomela Feb 5 '14 at 11:12
@JukkaSuomela: thanks a lot, I tried to find exactly this part of specification but gave up too early. – ssavec Feb 5 '14 at 11:25

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