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VBoxManage internalcommands createrawvmdk -filename d:\VMs\gCFRAW.vmdk -rawdisk \\.\PhysicalDrive4
C:\Program Files\Oracle\VirtualBox>VBoxManage internalcommands createrawvmdk -filename d:\VMs\gCFRAW.vmdk -rawdisk \\.\PhysicalDrive4
Oracle VM VirtualBox Command Line Management Interface Version 3.2.8
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RAW host disk access VMDK file d:\VMs\gCFRAW.vmdk created successfully.
You can also access specific partitions of host physical disk from VirtualBox, run the same command with additional switches,
VBoxManage internalcommands createrawvmdk -filename c: \Users \sunuser \.Virtualbox \VDI \ mydrive.vmdk -rawdisk \ \. \ PhysicalDrive4 -partitions 1,4
1,4 : are the partitions numbers of PhysicalDrive 0. It will allow you to access the first and fourth partitions of the physical drive.
This command also will create a VMDK file and you can access the drives as explained in first method.