Keeping my hard-drive off as long as possible

I'm not sure if this is strictly a hardware or software issue, but here goes:

My computer set-up is such that I have two external hard-drives striped together into a single drive that equals the capacity of my computer's internal drives. These drives are ONLY used by Time Machine, so are only required once an hour at most.

The drives generate quite a bit of heat (more than my MacPro!) and have their own fans, and with the UK's summer months on the horizon I'd rather have them off as much as possible. So I've set-up Energy Saver to switch off hard-drives whenever possible.

Problem is; "whenever possible" seems to actual mean "randomly". The drives are ONLY needed once an hour when Time Machine does its back-ups, yet they switch on and off almost continuously at non-fixed intervals. This of course renders the setting useless, as the drives are likely using more energy switching themselves on and off all the time, not to mention the increased wear and tear this involves.
The drives also spin-up any-time I get a file-open or file-save dialogue, which again is pointless, as I never open or save anything to them. The drives are even indexed by Spotlight (I've tried turning this off, no-joy), so they shouldn't need to spin-up even if I'm exploring them in the open-dialogue. The only time in fact that the drives should ever be active is if they are being written to, or I actually open one of the files on them, which only happens when Time Machine is being used.

Does anyone know of any-way I can change the behaviour of the drive so that it will only become active when Time Machine is running? Or some other solution? As I say; these drives are _only_ used as a Time Machine volume, except during back-ups I'd like them to remain off.

Cheers

Is it connected by USB perhaps?
External USB enclosures are notoriously poor when it comes to disk power management and often ignore OS preferences. Some new enclosures now come with dedicated power management software but not all do and most older ones don't.
Firewire connected externals don't often suffer with this issue and are a better choice is your looking for good power management for an external drive.

You may be able to find a 3rd party app/widget/script to control your drives.

They're actually both firewire 800 drives. The Maxtor OneTouch 1.5tb monsters, they're good drives but OS X seems to find switching them on and off all the time amusing, it's usually around 5 to 10 minute intervals but still far more frequent than I'd like.