Error:

I ran into the following error while converting a virtual machine on a VMWare ESX Server to OVF using the VMWare OVF Tool.

Disk Transfer Failed
Error: Unable to open NFC target disk

Googling suggested that a network problem or a permissions problem might be at fault. In fact, it turned out the problem was that someone had started the image on the ESX server mid-way through conversion. The OVF Tool cannot convert a running image — it requires a stopped machine. Stopping the vm made the problem go away.