I'm looking for a way to get physical diversity for backup storage. There
are friends and family who would gladly allow me to put a box in their
house, but I don't want to mess with their computers and all the ongoing
headaches of that. So my notion is to have an external hardrive with an
ethernet interface and a built-in secure FTP server. In this way, I could
hang the box onto their LAN, but not have to fiddle with their computer.
And, it needs to be relatively inexpensive.

There are a couple of such inexpensive devices, such as from TrendNet, but
the ones I've found have an FTP server, but don't support secure FTP (e.g.,
SFTP).

Can anyone point me to any such products?

Many thanks ...