01VirtualBox is not VMware
A VDI is a different container from VMDK or VHDX. UUID, snapshot overlays and the VirtualBox.xml that sits beside the disk do not travel. We convert the disk file you upload.
If you actually have an OVA appliance, use the OVA guide — that is a tar of OVF plus disks, not a raw VDI.
02Snapshots
A VDI that still depends on a snapshot overlay may not be a complete disk. Convert the merged disk. An incomplete chain fails with a diagnostic instead of a QCOW2 that looks plausible and boots garbage.
03Isolation
The worker identifies VDI, converts in a disposable environment, validates QCOW2, then offers a download that expires. Densem is not datastore storage.