sometimes you simply want to copy a vdi file and attach it to another virtual machine.
This will fail because in a VirtualBox Installation each vdi Harddisk must have an unique UUID. This command line changes the UUID of a VDI file.
VBoxManage internalcommands sethduuid /VMs/debian.new/sda.vdi
UUID changed to: f668f3e6-25b6-479c-bbe8-8120ba879849
Alternatively you can clone an vdi file:
VBoxManage clonehd /VMs/debian/sda.vdi /VMs/debian.new/sda.vdi --format VDI --variant Fixed
This also generates an UUID for new vdi file.