Backup disk partitioning strategy

I have several pcs, each with hdd with several partitions
(OS, data, sw, lib (e-books etc.), audio/video) and then there's
the pcs of other family members.

I'm now trying to come up with a reasonable solution to my
backup needs + being able to access data centrally
(avoiding having copies of files on a number of pcs). I purchased:
- a 1TB WD NAS and
- a 1TB seagate USB drive

I'm not clear on what would be the wisest way to implement the
solution:
(i)Have a "server" pc with data (including email &bookmarks), sw, lib,
audio/video partitions and upload/download files there directly and make
images of these partitions on the NAS and USB drive + sync
email & bookmarks from there
OR
(ii) Use the NAS to do the same as the "server" pc, include a
"backup" partition(for images) and duplicate the images only on
the USB drive ?

In case the NAS/ USB drives are used just for backup, is it enough
just to have one partition on each of these drives and separate info with
folders ?