I have a bootable USB drive with Ubuntu installed. I tried to boot it from VirtualBox.
lkraav's post on forums.virtualbox.org helped me to figure it out.
- Create a new VM
- Uncheck the "Boot Hard Disk" option
- 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.)
- Figure out the device name using "Disk Utility"(Under Application -> Utilities). Mine was /dev/disk1
- Create a vmdk file.
- Add an IDE Disk
- Choose the vmdk file just created.
- Boot your virtual machine and enjoy your Ubuntu OS.