I'd like to point out that in a
NAS (basic ones at least) you're drives will be stitched together to form one partition usually in a
JBOD or RAID array. Such that removal of a single drive would be worthless to be reused for data as it doesn't contain all the data, you would have to remove all the drives, connect them up to another controller that can handle JBOD/RAID setups, then get the configuration right.
This is even completely before file system, after all you couldn't use partial file systems no matter what type they were. NAS provide the filesystem itself, and manages connections to it so as to allow multiple clients efficiently.
I would recommend if you're looking towards expandability go with a 4 bay NAS not a 2 bay NAS. Most NAS would likely default to a JBOD array, which would allow for adding a drive (but not easily just replacing one).
I don't have any recommendations on which to get.
If you're still concerned about the filesystem, you may want to instead invest in a
SAN or a fileserver.