Cloning a PATA or SATA Hard Disk
There are two processes used by computer forensics examiners for making a bit-for-bit copy of a hard drive:
- A disk clone is an exact copy of a hard drive and can be used as a backup for a hard drive because it is bootable just like the original.
- A disk image is a file or a group of files that contain bit-for-bit copies of a hard drive but cannot be used for booting a computer or other operations.
The image files can also be different because they can be compressed, unlike a disk clone, which is not compressed. When cloning, the bit-for-bit copy is transferred to a second hard drive that is of equal size or larger than the source drive. Another difference is that specialized software, like EnCase, X-Ways, or FTK, is needed to view the contents of the image files. In general, image-viewing software is read-only, and files cannot be added. Nevertheless, some applications allow image files to be edited; WinHex, which is produced by X-Ways Forensics, is one such example.
The process of cloning a hard drive is a faster process than imaging a hard drive. The time difference between the two processes is substantial. Therefore, when a computer forensics examiner is working undercover or perhaps needs to obtain a copy of a hard drive and leave the computer with the custodian, then cloning the drive is more practical. On average, successfully cloning a SATA drive takes less than an hour. Of course, the time to clone depends on the size of the source hard drive and the cloning equipment being used.
One forensic cloning device used in investigation is the Disk Jockey PRO Forensic Edition (see Figure 3.9). The device is write-protected and allows the user to copy directly from a SATA or IDE hard disk drive to another SATA or IDE hard disk drive.
FIGURE 3.9 Disk Jockey PRO Forensic Edition
Before the investigation, all harvest disk drives must be sanitized. The Disk Jockey PRO has a function that performs a Department of Defense (DoD) seven-pass secure erase. When a new hard drive is removed from the packaging, it should be securely erased because an attorney might question a forensic investigator on this. Other devices, like the WipeMASSter Hard Disk Sanitizer from Intelligent Computer Solutions, are used solely to securely erase hard disk drives.
Before embarking on an investigation, it is also helpful to identify the specifications of the suspect’s machine (the make and model), where possible. This enables the investigator to research the computer that they will be working on and learn how to remove the hard drive. This might sound like common sense, but removing a hard drive from a Dell Inspiron 6400 laptop for cloning is very different from removing the drive from a Dell Latitude D430. The equipment required to clone each of these hard drives is also very different. A Dell Inspiron 6400 is relatively easy to remove, and then you can connect a SATA data cable and a SATA power cable. For a Dell Latitude D430 (or D420) laptop, the battery must be removed; then a thin cable must be removed from the hard drive and a rubber casing around the drive also must be removed. A special ZIF cable, ZIF adapter, and IDE interface cable are necessary to connect the 1.8-inch SATA hard drive to the Disk Jockey PRO, as in Figure 3.10. If possible, also try to predetermine the target computer’s operating system.
FIGURE 3.10 Cloning a hard disk drive with Disk Jockey PRO Forensic Edition
A simple Internet search for “removing the hard drive dell 430” will result in helpful documentation (including pictures) that Dell has made available online. In fact, Dell maintains a web page for most of its computer models that details hard disk drive removal. For other computers, manufacturers provide similar documentation. Removing the hard drive from the most recent iMac is a very involved process that requires some unique tools. Apple provides comprehensive instructions on the removal of hard drives from iMacs. YouTube.com also has numerous helpful videos to help the investigator.
The Disk Jockey has both a “Disk Copy” and a “Disk Copy (HPA)” cloning function. An investigator should first attempt to use the Disk Copy (HPA) clone function. This function makes a copy of the disk that includes the Host Protected Area (HPA). The Host Protected Area (HPA) is a region on a hard disk that often contains code associated with the BIOS for booting and recovery purposes. Manufacturers use the HPA to assist in the recovery process and replace the need for a recovery CD. An investigator should try to make a copy of this area because criminals have been known to hide incriminating evidence in this region of the disk. Sometimes the Disk Jockey PRO is unable to recognize and copy the HPA. When an error message appears on the Disk Jockey PRO’s LCD display, the investigator must then use the Disk Copy function instead of Disk Copy (HPA).
Alternative Copy Devices
The ImageMASSter Solo IV Forensic is a much more expensive device than the Disk Jockey PRO, but it has the ability to image two devices simultaneously. The investigator can select either a Linux DD file or an E01 image file.
Solid State Drives
A solid state drive (SSD; see Figure 3.13) is a nonvolatile storage device found in computers. Unlike on a hard drive, files on a solid state drive are stored on memory chips in a stationary layout of transistors, not on metal platters. In other words, a solid state drive has no moving parts—no read/write heads or spinning disks. Most solid state drives are flash memory NAND devices. It is important to know about these drives because they are growing in importance; they can be found in Chromebooks, the MacBook Air, and numerous personal computers today.
FIGURE 3.13 Solid state drive
In a single-level cell (SLC) NAND flash, each cell in the SSD has 1 bit. In a multilevel cell NAND flash, each cell has two or more bits. An MLC has higher density but generally requires more voltage than an SLC.
There are more than 60 SSD manufacturers, while there are very few hard disk drive manufacturers. There are numerous controller manufacturers who have different manufacturing requirements for SSD manufacturers. Therefore, this complicates the life of a computer forensics investigator, i.e. an SSD from one manufacturer can have different controllers with varying firmware. The proprietary firmware associated with the controller affects garbage collections, caching, wear-leveling, encryption, compression, bad block detection, and more.
Consider some examples of SSD controller manufacturers:
In many ways solid state drives are a more efficient alternative to hard disk drives, given their more efficient use of power; faster retrieval and storage of files; and greater resistance to environmental factors, including heat and vibration. Nevertheless, solid state drives suffer from ware-leveling. Ware-leveling is the process by which over time areas of a storage medium become unusable.
From a file storage perspective, solid state drives are very different from hard disk drives, and they do not use the traditional 512-byte storage sectors.
In terms of computer forensics, recovering deleted files on a solid state drive is more challenging as a result of the garbage collection process. Garbage collection is a memory-management process that removes unused files to make more memory available. Garbage collection is rather unpredictable with solid state drives and is particularly uncontrollable. Changes to files stored on a solid state drive can occur without warning, regardless of the best efforts of a computer forensics examiner. Garbage collection and other automated functions associated with an SSD mean that once a hash is forensically created for a hard drive, then when a hash is generated again on the same drive, the two hashes are unlikely to match, which is different with a HDD.
Unlike a hard disk drive, with an SSD, data must be erased before a write can occur; writes are completed in large blocks with high latency. Another difference is that the operating system does not keep track of the physical location of files; a file translation layer (FTL) is responsible for this. The File Translation Layer (FTL) maps a logical block address to a physical block address. TRIM is an operating system function that informs a solid state drive which blocks are no longer in use, which allows for high write performance. TRIM runs immediately after the Recycle Bin is emptied.
Random Access Memory (RAM)
Random Access Memory (RAM; see Figure 3.14) is volatile memory that is used for processes currently running on a computer. Its volatile nature comes from the fact that, when a computer is powered off, the contents of RAM are generally erased. However, if a system is powered on, RAM can provide a forensics examiner with a treasure trove of information, which can include Internet searches, websites visited, and possibly even passwords.
FIGURE 3.14 RAM chip
Redundant Array of Independent Disks (RAID)
A Redundant Array of Independent (or Inexpensive) Disks is commonly referred to with the acronym RAID. A RAID, is where two or more disks are used in conjunction with one another to provide increased performance and reliability through redundancy (see Figure 3.15). In the case of a RAID, reliability refers to fault tolerance, which means that if one component in a system, like a hard disk drive, fails, then the system will continue to operate. This kind of reliability is worth the investment for many critical systems in an organization. More recently, organizations have installed RAIDs to increase storage. Although RAID contains multiple hard disks, the operating system views the RAID as one logical disk with the use of hardware controllers.
FIGURE 3.15 RAID
From a computer forensics perspective, it is important to know that a computer may have multiple hard drives connected to it, all of which have evidentiary value. It is also important for an investigator to note the order in which each drive was added to the RAID and which drive adapter is connected to which drive (it can be confusing).