$ diskutil list
Take note of the disks listed. Now insert the disk you want to convert and run the above command again. It should show one more disk device. You can be sure that the added disk is the one you just inserted. You can skip this step if you already know which disk to convert.
Make sure the disk is unmounted but not ejected by executing the following command:
$ diskutil unmountDisk /dev/disk2
in this example I want to convert the USB stick listed as disk 2 to a VDI file.
Finally we need to convert the disk by using the following command:
$ sudo VBoxManage convertfromraw /dev/disk2 usb.vdi --format VDI Converting from raw image file="/dev/disk2" to file="usb.vdi"... Creating dynamic image with size 61872793600 bytes (59007MB)...
My USB stick has a size of 64GB. Make sure that you have enough disk space available for where you are going to save your virtual box disk image. 03/14/2016