Are you on a Mac? I've been using Apple's Time Machine and it is very fast. My setup is capture tethered to a MacBook Pro Retina with solid state drive which has an external USB3 solid state drive dedicated to Time Machine. The speed is amazing—I don't remember the exact speed of the backup but it's usually under a minute to backup a couple gigabytes. I do a backup when I feel I've done a bunch of captures that I wouldn't want to lose. Time Machine also does a backup every hour automatically. Since the interface and drive are so fast, I have never experience any interruption when Time Machine decided to do it's hourly auto backup.
I don't think that backup up to SD card is a good idea. Also, it's probably a slow transaction, right? If you are a pro shooter and have a busy set, you don't ever want to have to wait for a backup to finish or bog you down.
Also, a DigiPlate would solve your problem of your hard drives falling of the table.
Hope some of this helps!