I don't have or know anything about that drive, but I think pretty much all modern external drives will spin-down or "sleep" the drive after its been unused for awhile (like, 5 minutes). I don't think leaving it on will be any problem at all. Most new external drives don't even have an on-off switch. Does yours?
I've got a WD Passport Studio (Firewire/USB 2.5" external drive) that I leave on all the time. It goes into a low-power state, in this particular drive it's indicated by a slowly-flashing LED.