Q: Set OS X timeout for external USB drive spin-up?

I have a third party external USB (RAID) drive with slow spin-up time.

After some time of inactivity OS X asks the drive to go to sleep, which it does.

When the drive is accessed again it takes approximately 20 seconds to come on-line again. In the mean time OSX reports an error "Disk Not Ejected Properly" after about 10-15 seconds. After this, the disk is available to the OS again.

I am happy with the energy saving functionality and settings.

My question is: how do I increase the time-out OS X uses so the disks have enough time to spin up and come on-line?