Working for me. With some previous AC68R firmware versions back to Feb, I did have some frustrations, noted below. With current firmware it's been straight forward.
Maverick's 10.9.2,
AC68R, Firmware Version:3.0.0.4.374_5656,
Drive format Mac OS Extended (required for TimeMachine, I believe).
Portable USB powered drive (no power brick for drive).
Connected to USB 2.
In router, select:
USB application
Enable Time Machine On
Backup Path /mnt/sda2
TimeMachine Volume Size GB (0: Limitless)
On Mac menu bar: Time Machine, select Preferences.
Select Disk: "RT-A68R-6xxx" [in my case]
Then a Mac dialog pops up:
Admin Log-in: xxxx
User Pass: xxxxx
Then, select TimeMachine, Back Up Now. It works.
Once, on previous router firmware, when I was having a problem seeing the drive as an option to select in TimeMachine:
a) I did take the USB drive and connected it to the MacBook Pro and ran Disk Utility and Repaired Disk. (I don't know whether it helped, but it was a step I took.)
b) I re-connected the drive and restarted the router (unplugged, and replugged power).
I've used the same set up with a second AC68 at a friends house, setting up a 3TB desktop drive with power brick. There are 4 Macs with various OS versions, but mostly 10.8.4 or 10.8.6, backing up to that 3TB drive successfully. I recall using USB 2.
If the mount drive dialog doesn't come up, in menu bar select Go, and then Connect To Server.
Server Address:
afp://RT-AC68R-6xxx._afpovertcp._tcp.local [replacing xxx with the name of your disk or
afp://RT-AC68R-6028._afpovertcp._tcp.local]
That should prompt you for the log in Admin: [admin user name] and Password: [xxx]. The disk should then show up as a selectable device under Time Machine preferences.
When in doubt, a restart of the Mac doesn't hurt. Good luck.
