Disks spinning down and then up and then down.....

Here's a thing:

I have fairly high performning PC running Vista Ultimate 32 bit.

I have 3 internal hard drives (system, data & system backup) with a 4th,
effectively internal, e-SATA drive as data backup.

My computer power settings are set to Performance and the disks are not
supposed to spin down for 20 minutes, except...... that they do do constantly.

It's a minor I know irritation but I'm often having to wait a few seconds
while a disc (one of the 3 internal ones) which has just spun down gets back
up to speed to change settings, write to the disk, close down etc etc.

Any suggestions?

Thanks.

Steve

Specs:

Vista Ultimate 32 bit
Asus P5B Deluxe motherboard
Core 2 Duo 6700 clocked @ 3GHz
4 GB Corsair 8500 Dominator RAM clocked @ 900MHz
Gigabyte 8800GTS 640MB
Western Digital 150GB Raptor SATA
Western Digital 500GB Caviar SATA2
Creative XFi Xtreme Music
Samsung SH-W183 DVD-RW SATA
Enermax 850W Galaxy PSU
Akasa Mirage 62 case
plus e-SATA Western Digital 500GB Caviar SATA2