Thanks to Al and Frank for responding so quickly. Leaving the HD set to never sleep is fine, however I did test all the USB devices and found that they are not the problem (at least not directly). Not waking up from sleep mode (perhaps in sleep mode for 30 minutes or more) has occurred regularly; if it's in sleep mode for 5 minutes or so it wakes up). When I tested the sleep mode without any USB devices connected, the machine did not wake up. I also updated some drivers and that did not help. So for now I have the HD set to "never" in the Energy Saver of System Preferences. Thanks, Rob