I have two 500GB hard drives. It seems like a waste of energy to be running both at the same time, so what I’m thinking of doing is occasionally initialising the spare drive, backing my stuff up on it, and then de-initialising it again, so that it does not run automatically when I start the computer.
Is this a reasonable thing to do? By backing stuff up on the drive and then leaving it dormant until the next backup (or until I need the backup), am I doing anything stupid? I’m just checking before I embark on this.
Yep if the power cable is connected to it then the energy saved will be most likely none, but at leas you will preserve your backups from being damaged or corrupted when using the laptop. So in the end i see no problem in disabling it when not needed, at least your backup will be more secure.
It’s a laptop. 
In the end I went ahead with it, working very well. I think you might be right, Stephen, in that it was spinning all along.
You could also consider a front-loading disk caddy that way you can securely store the disk when not in use or just shove the HDD into the PC when needed.
It’s not external though. Both drives are inside and connected to the motherboard. It’s just that one is completely empty and not initialised. Within the Computer Management panel in Vista I can right-click it and “initialise” it. I guess I’m asking if “de-initialising” a drive is the same sort of thing as disconnecting an external HDD.
I do this all the time. No need to constantly have external hdd’s connected to your PC at all times. My PC won’t even boot when the external hdd is connected and turned on 
I wouldn’t think so. If a hard drive has power plugged into it then the drive is spinning whether the data cable is connected so as to allow it to be accessed by the computer (so as to be able to initialise/use it) or not.
The only way to not have the drive drawing power is to disconnect the data cable.
There is also a BIOS setting for being able to power down parts of the computer when they are not being used but to the best of my knowledge having it power down hard drives means that all the hard drives get powered down.