Hard drive disconnects itself improperly-Hard disk sleep?

Hello,

I have recently bought a 2011 Macbook Pro and I am facing a problem with an external Verbatim LCD hard drive.

It seems like when the drive is inactive for 10 minutes or so, it gets disconnected improperly, I get an error message and the drive disappears from the system until I unplug it and plug it back in.

Also I have noticed that when the disk is in use , it does not disconnect. I tried leaving playing a movie to see what will happen and everything was ok. When I stopped the movie and left the Macbook Pro it disconnected itself after a while.

I know this may be bacause of the option in the Power Saver "Put hard disks to sleep if possible".

So, I have decided to untick this option and up to now everything seems fine again.

I just wonder, should I leave this option on or off? And if this option is there, why does it cause such a serious problem like disconnecting a hard drive?

This certainly should not be happening!

I would appreciate any information on that.

Thank you in advance!