I'm serious about the powersupply because I've had those probs from my failing powersupply. It could also be a cable not secure 100%. If you want to check the power supply prob them unplug as much from your computer was possible to unburden your power supply. (extra hdd, dvd/cd, vid card (if u have onboard to use instead), usb stuff..).
For some reason lately I've been super suspicious of power supplies. I seem to blame them for everything, its just a phase
If this is happening to both hdds then it could be anything from your cables, to powersupply, to motherboard (could even be a piece of hardware trying to use irq 14 or 15 (if IDE)), to cross talk (components interferring, check cables). If it only happens to the one hdd then it could be the hdd is failing, download something like HDD health (www.panterasoft.com) and check the status of it. (smart needs to be enabled in your BIOS, usually is by default now, if HDD health works then its enabled).