This morning, my Mac would not start. It sat for a long period of time on the light blue screen but wouldn't move on. There was no hard drive activity during most of this time either. After unplugging USB and FireWire devices one at a time, I tracked it down to my Drobo. I unplugged the power from the Drobo for a few seconds to restart it. After it restarted, and I plugged it back into the Mac, everything works fine.