No problems here with Toshiba HDD in UBS 3.0 case plugged into the front USB port. Unplugged it before updating to 56 beta, mounted fine and working.
Looks like it's trying repeatedly to mount it, but failing, with no report as to what is failing.
I can't reproduce the issue here with my own HDD, and I'm not familiar with the details of the USB implementation (the code is quite complex, and undocumented). See if you can reproduce the problem with the stock Asus firmware, and if yes, report it on the ASUSWRT - Official Firmware forum here so they can look into it.