If you can stagger the spin-up of the HDDs, you should be fine. Just be aware that if the HDDs all start at the same time, the current may exceed the PSU rating briefly, and you may find some drives not available. If you have a UPS and won't be turning the system off very often, you'll probably be ok with the 250W, but check the 5V and 12V rail specs, and maybe do some testing. Don't rely on the total power ratings, as they don't apply on system start in many cases.