Time Machine back up EXTERNAL drive?

Hi, I was wondering if Time Machine can back-up external drives ... e.g ... could I have a 1TB drive as the target and have TM back-up bothmy iMac's HDD andan external HDD I already own of 500GB.

Reason I ask is that the 500GB drive is my current TM target and it's full - and only 18Gb left on the internal too!

I called Apple support and they were ... indecisive ... sometimes it works, sometimes it doesn't apparently. Either way, it's not supported.

I ask this as my 500GB drive is perfectly good and I'm looking for a way to increase my memory AND improve my back-up without going to the expense of buying two 2TB drives to set-up RAID1.

Thanks all,
Shane.