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This chapter is from the book

This chapter is from the book

Removable Media

Most removable media also have partitions, but they use the same structures that hard disks use. The exception to this rule are floppy disks that are formatted for FAT12 in a Windows or UNIX system. They do not have partition tables, and each entire disk is treated like a single partition. If you image a floppy disk, you can directly analyze the image as a file system. Some of the small USB storage tokens (sometimes called ’thumb drives’) do not have partitions and contain one file system, but some of them do have partitions.

Larger removable media, such as Iomega ZIP disks, do have partition tables. The partition table on a ZIP disk will depend on whether it has been formatted for a Mac or a PC. A PC-formatted disk will have a DOS-based partition table and by default will only have one partition in the fourth slot.

Flash cards, which are commonly used in digital cameras, also typically have a partition table. Many flash cards have a FAT file system and can be analyzed using normal investigation tools. Here is DOS-based partition table from a 128MB flash card:

# mmls -t dos camera.dd
DOS Partition Table
Units are in 512-byte sectors
     Slot    Start        End          Length       Description
00:  -----   0000000000   0000000000   0000000001   Primary Table (#0)
01:  -----   0000000001   0000000031   0000000031   Unallocated
02:  00:00   0000000032   0000251647   0000251616   DOS FAT16 (0x06)

Putting flash cards in a USB or Firewire reader and using dd in Linux can easily image them.

CD-ROMs are more complex because there exist many possible variations. Most CDs use the ISO 9660 format so that multiple operating systems can read the contents of the CD. The ISO 9660 naming requirements are strict, and there are extensions to ISO 9660, such as Joliet and Rock Ridge, which are more flexible. CDs are complex to describe because one CD may have data in a basic ISO 9660 format and in a Joliet format. If a CD is also an Apple hybrid disc, the data could also be in an Apple HFS+ format. The actual content of the files is only saved once, but the data are pointed to by several locations.

Recordable CDs, or CD-Rs, have a notion of a session. A CD-R can have one or more sessions on it, and the purpose of the sessions is that you can continue to add data to CD-R more than once. A new session is made each time data are burned to the CD-R. Depending on the operating system in which the CD is used, each session may show up as though it was a partition. For example, I used an Apple OS X application to create a CD with three sessions. When the CD was used in an OS X system, all three of the sessions were mounted as file systems. When the CD was used in a Linux system, the last session was the default session to be mounted, but the other two could be mounted by specifying them in the mount command. The readcd tool (http://freshmeat.net/ projects/cdrecord/) can be used to determine the number of sessions on a CD. When the CD was used in a Microsoft Windows XP system, the system said it was invalid, although Smart Project’s ISO Buster program (http://www.isobuster.com) in Windows could see all three sessions. Different results may occur if the multiple session CD was created from within Windows. It is important with CD-Rs to use a specialized CD analysis tool to view the contents of all sessions and not rely on the default behavior of your analysis platform.

Some CDs also contain the partition systems of the native operating system. For example, a hybrid CD is one that is in an ISO format and an Apple format. Inside the session are an Apple partition map and HFS+ file system. Standard Apple investigation techniques can be applied to these disks. For example, here is the result of running mmls on hybrid disk:

# mmls -t mac cd-slice.dd
MAC Partition Map
Units are in 512-byte sectors

     Slot    Start        End          Length       Description
00:  -----   0000000000   0000000000   0000000001   Unallocated
01:  00      0000000001   0000000002   0000000002   Apple_partition_map
02:  -----   0000000001   0000000002   0000000002   Table
03:  -----   0000000003   0000000103   0000000101   Unallocated
04:  01      0000000104   0000762559   0000762456   Apple_HFS

Many bootable CDs also have a native partition system. Sparc Solaris bootable CDs have a Volume Table of Contents structure in the ISO volume, and Intel bootable CDs can have a DOS-based partition table at the beginning of the CD. These structures are used after the operating system has been booted from the CD and the code required to boot the system is in the ISO format.

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