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1. Create a new VM
   
2. Uncheck the "Boot Hard Disk" option
   
3. Connect the USB disk to your iMac, and choose "ignore" if your system says "The disk you inserted was not readable by this computer".(Because iMac does not understand ext3/4 file systems.)
4. Figure out the device name using "Disk Utility"(Under Application -> Utilities). Mine was /dev/disk1
5. Create a vmdk file.

   No Format
   ~/VirtualBox VMs$ VBoxManage internalcommands createrawvmdk -filename MyUsbUbuntu/WD80.vmdk -rawdisk /dev/disk1 RAW host disk access VMDK file MyUsbUbuntu/WD80.vmdk created successfully.

6. Add an IDE Disk
   
7. Choose the vmdk file just created.
   
8. Boot your virtual machine and enjoy your Ubuntu OS.

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