One of main reasons for selecting the RT-N56U was that it came with two USB 2.0 ports. Since routers are never turned off, the USB ports could be used for backing up your computers. It is my belief that backups should always be done to a second disk drive. (Installed either on the computer itself or as an external device, in this case the router.) To accomplish that, I bought a 3TB Western Digital USB hard drive, which plugged into the router's USB port.
For the backup program, I used Simple Backup. This is an excellent back-up program. Though an excellent program, it proved to be quite finicky when it came to backing data onto the USB HD drive attached to the router's USB port. Periodically the back-ups would fail with permission/access error messages. Worked without issue when backing-up to a USB device that was directly attached to the computer.
I have Simple Backup working as expected with the RT-N46U router. The following tweaks may have resulted in getting Simple Backup to function correctly.
1. Mount the USB HD Drive. See the /etc/fstab entry below.
#Mount Western Digital Passport Drive On Router
//rt-n56u/sda1 /media/My_Passport cifs rw,sec=ntlm,guest,uid=1000,gid=34,iocharset=utf8,nofail 0 0
2. Change the format of the USB HD to EXT3.
Why EXT3? Turns out that the Asus RT-N56U does not recognize EXT4. The fact that the router does not recognize EXT4 is "hidden" in the manual. Had to actually read the manual to find that out. Grumble.
Check to see if your router supports EXT4.
Furthermore, The Western Digital USB HD, as purchased, came formated with Microsoft's NTFS. Under NTFS, I would receive periodic permission/access error messages when connected through the router's USB port. Since changing the file system to EXT3, these issues have disappeared.
With the tweaks identified above, Simple Backup is now operating reliably. These tweaks may also prove to be beneficial with other modems that offer USB ports where a USB hard drive can be attached.