qemu-img convert fails to convert, generates a 512byte file output

I have a Vmware image, so I have files like 'Ubuntu.vmdk', want to convert to VirtualBox .vdi format using qemu, the first stage of extracting the image with 'qemu-img convert Ubuntu.vmdk output.bin' just generates a 512byte file:

{quote}
# Disk DescriptorFile
version=1
CID=36be9761
parentCID=ffffffff
createType="twoGbMaxExtentSparse"

# Extent description
RW 4192256 SPARSE "Ubuntu-s001.vmdk"
RW 4192256 SPARSE "Ubuntu-s002.vmdk"
RW 4192256 SPARSE "Ubuntu-s003.vmdk"
RW 4192256 SPARSE "Ubuntu-s004.vmdk"
RW 4192256 SPARSE "Ubuntu-s005.vmdk"
RW 4192256 SPARSE "Ubuntu-s006.vmdk"
RW 4192256 SPARSE "Ubuntu-s007.vmdk"
RW 4192256 SPARSE "Ubuntu-s008.vmdk"
RW 4192256 SPARSE "Ubuntu-s009.vmdk"
RW 4192256 SPARSE "Ubuntu-s010.vmdk"
RW 20480 SPARSE "Ubunt
{quote}

No stack trace or other output was found.  Anything I can add (other than the 20G VM image to reproduce and I'll be happy to provide)