Be aware that the USB speed on routers -- that includes the RT-N66U (which is being toted as "awesome" by fanboys around the world, sigh) -- is atrocious for two reasons:
1) There is no decent offloading support for USB I/O on those routers. The SOCs are not well-known for these types of features. They do not perform nor are engineered anywhere similar to that of a standard PC, or even a PC made 10 years ago. I've already seen numerous reports on the linksysinfo.org forum of people having "strange problems" with their USB-attached hard disks, and in most cases the problem turns out to be wonky enclosures or enclosures which are attempting to be powered purely off the USB bus (not enough to properly spin up a 3.5" drive, and a lot of the time can't spin up a 2.5" drive either).
2) These units are run by a very old Linux kernel which has a horrible performance bug pertaining to sendfile(2) support. Out-of-the-box, these firmwares use Samba, and the manufacturers moronically configured Samba with use sendfile = yes
. Disabling sendfile (a one-line change) results in a 30-60% speed increase as a result. (Upgrading the kernel is not an option, by the way, because of the wireless driver being a binary blob -- thanks Broadcom!)
As such, I recommend you do not
use your router as a NAS. I recommend you purchase a dedicated product that does the job. You need something that also offers support -- I cannot tell you how many of these fly-by-night proclaimed "consumer NAS" devices fail miserably under certain conditions where drives begin to misbehave or act oddly. So vendor support is very important.
The problem, as I see it, is a financial one. If you really want something that has native Ethernet capability along with CIFS/SMB or AFP, you're going to need to consider a dedicated device. For you, I would strongly recommend a Drobo product (they do make some that have gigE Ethernet, and they make an add-on module for some models which lack it). However, these are not within your very limited price range.
And finally, never forget: do backups
. Do not assume just because you have a NAS with 2 disks in a RAID-1 (mirror) model that you're safe -- you're not. DO BACKUPS
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.