You can duplicate hard disk image files on the same host to quickly produce a second virtual machine with the same OS setup. However, you should only make copies of virtual disk images using the utility supplied with Oracle VM VirtualBox. See Section 7.23, “VBoxManage clonemedium”. This is because Oracle VM VirtualBox assigns a UUID to each disk image, which is also stored inside the image, and Oracle VM VirtualBox will refuse to work with two images that use the same number. If you do accidentally try to reimport a disk image which you copied normally, you can make a second copy using the VBoxManage clonevm command and import that instead.

Note that Linux distributions identify the boot hard disk from the ID of the drive. The ID Oracle VM VirtualBox reports for a drive is determined from the UUID of the virtual disk image. So if you clone a disk image and try to boot the copied image the guest might not be able to determine its own boot disk as the UUID changed. In this case you have to adapt the disk ID in your boot loader script, for example /boot/grub/menu.lst. The disk ID looks like the following:

scsi-SATA_VBOX_HARDDISK_VB5cfdb1e2-c251e503

The ID for the copied image can be determined as follows:

hdparm -i /dev/sda