A:
It depends on your router. I don't recommend using USB2 for backups but USB3 is a worthy option if your router has a USB3 port. One thing to realize, depending on your router, some drives will spin down when not in use and you would want to make sure you "wake up" the drive before attempting a back up. This might be as simple as engaging the drive first and then doing a back up or try a script.
Also, some routers may not let an external drive spin down or go into sleep mode which means its always 'hot" or active. This can be good or bad depending on your needs and willingness to bun a tiny amount of electricity continuously.