Techo question

I've just noticed my husband's hard drive light flashing on and off, regularly, about once a second. No sound coming from the drive. No programmes running. I have a sinking feeling that I read somewhere that it means imminent hard drive failure (nooooooooo!). Anyone got any ideas what could be causing it?

As a comparison, I have the same kind of computer, and mine stays off unless the computer is actually doing something...