Keeping the external HD on standby?

I have a WD MyBook (in Raid 1 configuration) from my MacBookPro.
After a period of inactivity, it clicks off into a standby mode: just as I expect and want it to do.
However, after every couple of minutes in standby, and with no request from me, it fires up again.... and then down...... etc.
I am unclear if it is just the MBP needing to check that the HD is still there, and demanding a response from it, or the HD itself going through some routine.
Or some application doing the 'prodding'?
All this unnecessary up-ing and down-ing must be shortening the HD's life, mustn't it??
How do I get it to stay asleep until I actually want to access it??
Many thanks!

It is happening with my new iMac too.
I put it to sleep, it wakes ups from nothing after a couple hours, them sleeps again. Would like to know the reason for this insomnia. I also have 2 external drives in this machine.

Most mac computers do a routine and constant systems check, i.e battery health (which you can see by holding alt and click the battery status), it's good in a way for your computer but if I were you i'd keep your HDD unplugged until you need it. As for IceB i'd get your iMac checked over