Cypher Technology Ltd V2.6 (03) December 2011
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5.1.9
Media history and fragmentation
When a file is created on a blank piece of media, the storage space is generally
allocated from the beginning of the data area as continuous or contiguous space. Once
several files have been created and then a few have been deleted “holes” appear on the
file data area where the old file data resided. These holes will eventually be overwritten
with new file data as new files are created, but because these new files may be of
different sizes to the old files, one new file may have its data spread across the card at
various locations. The more a flash card or drive is used, the more fragmented it
becomes. This gives valuable information about the use of the media and the time of
creation or modification of a file.
The Master Copy preserves all this information as it provides an exact snapshot
of the state of the card when the copy is taken and because it leaves all the card
data unchanged within the backup. The Master Copy is therefore the most
complete evidential backup.
5.2
Disk Verify Read
Most backup programs simply copy the data to the backup medium and assume it has
been written correctly and can be re-read. With optical disks this is particularly
dangerous. PUMA dramatically reduces the chance of producing unreadable disks by
performing a verify read of the disk after it has been written. The verify pass itself,
involves re-reading every data bit from the flash card and checking it against every bit
on the Master.
5.3
Working Copy (file copy)
The Working Copy is a full copy of the directory tree of the flash card or USB drive. This
means all files and directories are copied. Any file types can be copied. Hidden and
system files are copied.
For the files on the flash card to be recognised by PUMA, the card must be formatted
with a FAT file system.
The Working Copy disk uses an ISO9660 filing system and is recognised by most
computing platforms. The system also implements the Joliet extension to the ISO 9660
standard which allows for long file names.
Where RAW image files are detected, PUMA can automatically convert them to a JPEG
compressed image file with a compression ratio of about 7:1. The file has the same root
name as the original, so that dcn0005.nef becomes dcn0005nef.jpg. For most picture
content it will not be possible to readily observe any difference in quality between the
JPEG and RAW images. This feature is disabled by default, but can be enabled in the
Menu (see above).
A wide range of RAW file formats are supported and these are listed in the Appendix.
5.4
Card and Disk Checksum
PUMA produces a 32 bit checksum of the flash card and Master Copy file. The
checksum is a unique number derived from all the data on the card or from the whole
image file on the Master. As such the checksum will always change if any data changes