It has to do with the ability to recover from accidentally formatting
the card or (very much less likely) development of read errors in the
directory.
A FAT16 or FAT32 formatted card/disk consists of a directory containing
file names, a pointer to the address where that file begins and a count
of the number of storage clusters used (a cluster is the smallest
storage allocation unit and consists of a group of 512 byte sectors, the
smallest storage unit on the disk) For FAT16 format (CF cards of 2GB or
less) cluster size can range from as small as one sector (512 bytes) to
as large as 64 sectors (32KB). For FAT32, clusters are always 8 sectors
and 4KB.
When FAT format files are written they are initially written
sequentially. If you delete a file it opens a hole in the chain of
storage locations. Depending on the storage allocation logic in use by
your camera that hole may or may not be used when it comes time to store
the next image. If it's smart code it will not reuse that space until
the card is otherwise full. If it's not so smart or if you've filled
the card from beginning to end, storing another image requires finding
the space freed by your deletions. But it's unlikely that the size of
the file about to be written is exactly the same size as the original
file that had been in that space. If will either be smaller or larger.
If smaller the original space will be occupied by the new file plus a
tail end fragment of the old file. If larger the old space will be
occupied by a portion of the new file and the remainder will be
allocated to another chunk of empty space that might be anywhere.
Furthermore, that space may not be contiguous but may in turn be broken
into many smaller non-contiguous chunks. The directory also keeps track
of these non-contiguous storage areas via use of a linked list that
tells which clusters are used for that file and where they are all located.
When you format a disk or CF card on your computer you can do either a
full format or a "quick" format. A full format completely erases all
the directory information and all of the data by writing a fixed bit
pattern into all storage locations. It may also read back the erasure
data pattern to check the health of each sector. A quick format (what
the camera does) only erases the directory information. All of the file
data still exists but the list of file names and where they start and
end has been lost.
If you were to accidentally format your CF card or if the card developed
errors in the directory a smart utility can recover everything if all of
the files have been written sequentially. It doesn't have any pointers
to know start or end positions but, if it knows the format of the JPEG
or raw files, it can recognize the starting position of a new file by
recognizing certain file header or descriptive information at the
beginning of the file that is independent of the actual image data. It
then infers the size of the file by searching for the next image header.
If there have been no erasures the end of one file will be marked by
the beginning of the next and there's no ambiguity. The original file
name will not be known but the file will be successfully recovered. If
there have been erasures but that space hasn't been reused then the
recovery utiliyt will recover the file that you thought was deleted. If
the space has been reused that new image will likely be corrupted unless
it just happens to exactly fit the old space allocation. If smaller it
may pick up some piece of the image previously stored there. If larger,
other pieces of the image will be stored in one or more non-contiguous
locations and there is no way to know how to reconstruct them.
Fortunately, this has only happened to me once. I picked up the wrong
CF card, put it into the camera and realized about 1/2 second too late
that I was formatting the wrong card. Because I had manually erased a
number of images on that card the recovery utility only successfully
completely recovered a subset of images. Fortunately the couple that I
really needed were intact.
Lesson learned. Never erase images in-camera unless absolutely required.
Chuck Norcutt
John Hudson wrote:
> Someone posted a message recently opining that unwanted image files should
> not be deleted in camera.
>
> What was the reason for suggesting that unwanted images should not be
> deleted by exercising the trash-can button on the camera?
>
> jh
>
>
>
--
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