In my last post, I discussed using the Partition/Diagnostic Event Log in Windows 10 for USB device tracking.  Specifically, we looked at the volume boot record (VBR) available in this event log and what it means to a forensic investigation.  In this post, I’ll continue by examining the device master boot record (MBR) that is available in this event log and discuss how it can be leveraged in a forensic examination.

Event ID 1006 of the Partition/Diagnostic event log contains a field for the MBR of a device that was connected to the system. This field is used to store a hexadecimal string of the entire MBR of the device at the time the event record was created.  The MBR often contains boot code, the disk signature, and the disk partition table.  In addition to the partition table embedded in the MBR, the raw partition table bytes are included as a separate field in Event ID 1006.  It’s also worth noting that GPT disks are handled differently than MBR disks when it comes to this event log.  I’ll save the partition table and differences in GPT handling for a later discussion and will focus on the disk signature from an MBR drive in this post.

The \MountedDevices subkey has historically been used to determine the last drive letter and/or the volume GUID associated with a removable device by correlating the serial number or ParentIdPrefix stored in the \DosDevices\X: or \??\Volume{GUID} value data with its corresponding device.  This is well documented (here, here, here, and many other places).  However, the  value data available in the \MountedDevices subkey for a fixed device (such as an external hard drive) stores the disk signature of the device instead of the serial number or ParentIdPrefix.  This means that determining the previous drive letter of a fixed device using the \MountedDevices subkey requires that you know the disk signature of the fixed device.

The disk signature is a 4-byte value located at decimal offset 440 of the MBR and is easily obtained from the external device itself, but it’s difficult to come by without access to the device.  This is why obtaining the MBR from Event ID 1006 is so valuable – it provides the examiner with a means to determine a device’s disk signature without access to the device itself.

Disk Signature Embedded in Event ID 1006

By saving the value of the MBR field (pictured above) to a new file, you can use any hex editor or tool capable of parsing raw MBRs to obtain the disk signature from the MBR.  After determining the disk signature, you can then try to match the disk signature up with the value data in the \MountedDevices subkey to find a drive letter with which the disk was associated.  Another option would be to use a tool such as USB Detective that handles the extraction, parsing, and correlation of all of this information for you.  Regardless of your approach, the Partition/Diagnostic Event Log should be included in your USB device investigations.