One way to move a virtual machine or just as a backup solution is to create a copy of your virtual machine as an appliance. This appliance can then be imported back to VirtualBox or other compatible virtualization platforms such as VMware or QEMU.
VirtualBox's virtual machines can be exported to the Open Virtualization Format appliance format into a folder (ovf) or an archive file (ova).
Newer version is usually better but might not be compatible with older virtualization software.
.ovf exports to multiple files within a folder (each for disk, metadata etc) while .ova exports to a single file of a compressed .ovf format folder.
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