The Netgear NAS DUO can supposedly back up to attached USB storage so you may get that to work. I've not tried it though.
That's still the wrong way round. Graham is looking for a drive that can be a NAS on an Ethernet and also a slave device on a USB connection. USB ports on NAS drives are master ports to connect an external drive or stick to.
My best suggestion would be to get a NAS with an extension USB, and a portable drive to swap between the NAS and the Humax. However, if you are happy to run the MSP, I would have thought an external drive attached to the Humax would do all you need without an additional NAS.