I have an R6400 and want to connect a HDD via USB as a NAS drive

> I have a Nighthawk R6400 router [...]

   R6400[v1] or R6400v2?  Look for "Model" on the product label.
Firmware version?

> [...] When I look at the Netgear Router page, under Readyshare, the
> Readycloud button does not appear (see photo). [...]

   I don't trust Netgear router firmware with my stored data, so I don't
use this stuff, so I don't pay close attention, but I got the impression
from my readings here that Netgear was dropping (had dropped?)
ReadyCLOUD support from recent firmware versions on some (all?) its
routers.  This could be an artifact of my usual false-memory syndrome,
but, if you loaded an older firmware version and the feature reappeared,
then that would be suggestive.  I'm too lazy to scan all the firmware
release notes to see if anything relevant could be found that way.

> [...] By convention NAS usually means Ethernet. That's not USB.

   Sigh.  "By convention", in this context, NAS means Network-Attached
Storage.

      https://en.wikipedia.org/wiki/Network-attached_storage

   A purpose-built NAS server typically uses a high-speed interface
(Fibre Channel, SCSI, SAS, SATA, ...) to communicate with its storage
devices (disks, et al.), and an Ethernet or other network interface to
communicate with its client devices.

   Some router models offer some NAS capability using a USB interface to
a storage device, and the usual Ethernet interface for communication
with a client.  That is what's being discussed here.

   Some of the participants in these forums seem to believe (and enjoy
telling others) that "NAS" means a box whose label says "NAS", rather
than a gizmo which attaches a storage device to a network.

   Others believe that any gizmo which attaches a storage device, even a
USB-connected storage device, to a network, like, say, a Netgear
R6400[vX], constitutes network-attached storage (NAS).  I have no
trouble with this functional definition (because I can read), but some
people seem to, which leads to "help" like:

> [...] By convention NAS usually means Ethernet. That's not USB.

   A USB-connected disk, by itself, is not (normally) NAS.  A
USB-connected disk, when connected to a computer with the right
programming (and a network interface), can be, literally, exactly,
network-attached storage.

   But I don't expect to persuade the local "experts".

   It's true that a purpose-built NAS server which does not use USB as
its storage interface will typically have faster hardware, and
software/firmware which is more reliable and has more features (like
RAID, for example) than a consumer-grade Netgear router-as-NAS.  But,
network-attached storage is still network-attached storage.  For some of
us, at least.