File that works with qemu-img, and not vboxmanage:

peter:/mnt # qemu-img info /mnt/bcnasvm1/bcvm01-00/bcvm01-00_1-000001-flat.vmdk
image: /mnt/bcnasvm1/bcvm01-00/bcvm01-00_1-000001-flat.vmdk
file format: raw
virtual size: 60G (64424509440 bytes)
disk size: 12M

peter:/mnt # vboxmanage showhdinfo /mnt/bcnasvm1/bcvm01-00/bcvm01-00_1-000001.vmdk
VBoxManage: error: Could not find an open hard disk with location '/mnt/bcnasvm1/bcvm01-00/bcvm01-00_1-000001.vmdk'
VBoxManage: error: Details: code VBOX_E_OBJECT_NOT_FOUND (0x80bb0001), component VirtualBox, interface IVirtualBox, callee nsISupports
Context: "OpenMedium(Bstr(pszFilenameOrUuid).raw(), enmDevType, AccessMode_ReadWrite, fForceNewUuidOnOpen, pMedium.asOutParam())" at line 210 of file VBoxManageDisk.cpp

peter:/mnt # vboxmanage showhdinfo /mnt/bcnasvm1/bcvm01-00/bcvm01-00_1-000001-flat.vmdk
VBoxManage: error: Could not get the storage format of the medium '/mnt/bcnasvm1/bcvm01-00/bcvm01-00_1-000001-flat.vmdk' (VERR_NOT_SUPPORTED)
VBoxManage: error: Details: code VBOX_E_IPRT_ERROR (0x80bb0005), component Medium, interface IMedium, callee nsISupports
Context: "OpenMedium(Bstr(pszFilenameOrUuid).raw(), enmDevType, AccessMode_ReadWrite, fForceNewUuidOnOpen, pMedium.asOutParam())" at line 210 of file VBoxManageDisk.cpp


File that works with neither qemu-img nor vboxmanage:

peter:/mnt # qemu-img info /mnt/bcnasvm1/bcvm01-18/bcvm01-18-flat.vmdk
qemu-img: Could not open '/mnt/bcnasvm1/bcvm01-18/bcvm01-18-flat.vmdk': Operation not permitted

peter:/mnt # vboxmanage showhdinfo /mnt/bcnasvm1/bcvm01-18/bcvm01-18-flat.vmdk
VBoxManage: error: The medium '/mnt/bcnasvm1/bcvm01-18/bcvm01-18-flat.vmdk' can't be used as the requested device type
VBoxManage: error: Details: code NS_ERROR_FAILURE (0x80004005), component Medium, interface IMedium, callee nsISupports
Context: "OpenMedium(Bstr(pszFilenameOrUuid).raw(), enmDevType, AccessMode_ReadWrite, fForceNewUuidOnOpen, pMedium.asOutParam())" at line 210 of file VBoxManageDisk.cpp

peter:/mnt # vboxmanage showhdinfo /mnt/bcnasvm1/bcvm01-18/bcvm01-18.vmdk
VBoxManage: error: Could not find an open hard disk with location '/mnt/bcnasvm1/bcvm01-18/bcvm01-18.vmdk'
VBoxManage: error: Details: code VBOX_E_OBJECT_NOT_FOUND (0x80bb0001), component VirtualBox, interface IVirtualBox, callee nsISupports
Context: "OpenMedium(Bstr(pszFilenameOrUuid).raw(), enmDevType, AccessMode_ReadWrite, fForceNewUuidOnOpen, pMedium.asOutParam())" at line 210 of file VBoxManageDisk.cpp

But qemu-convert version 0.11 pretends to work with this file, and creates a bad image. The newer qemu-img convert does the same as the above.

See Bug 548723 - qemu-img convert silently corrupts when converting from VMDK 4 to raw
    https://bugzilla.redhat.com/show_bug.cgi?id=548723