The only way to know for sure if a UPS is fast enough for your drives is to test it. When the UPS is charged, unplug it from the wall and see what happens. If the drive(s) stay on, you are probably fine. If they click off then spin back up, you have a problem.
You could always make your own online UPS (though I don't really recommend it). You need a large deep cycle battery, an inverter big enough to handle everything that you might have plugged in, and a charger for the battery that can also supply enough power to charge the battery and power anything you might have connected to it at the same time. Hook up the inverter and the charger to the battery, turn both of them on, and you should be all set. I don't recommend it because most battery chargers are not designed to charge a battery and run an inverter at the same time. They usually have 2-4 distinct modes (constant current, constant voltage, trickle charge, etc) that might or might not be able to power the inverter, and the mode detection circuitry might be confused by the extra load. Also, cheaper inverters put out noisy power (modified sine wave) that might not be suitable for some devices and will cause your magsafe adapter to get very warm.
