Artie,
All my data is now on one 3TB drive called Main Hard Drive. ON that drive there is about 2TB of data.

I don't need snapshots of all that 2TB, I could be happy if time machine just backed up some important folders which are well unter 500GB.

But how?

TM options shows all hardrives and partitions on the computer. I tick to exclude the partitions I don't need, but it does not offer the ability to exclude specific folders on Main Hard DRive.

It seems you have to offer up to the TM Gods one hard drive in full, am I wrong? I would love to just offer up a few specific folders from Main Hard Drive and test it out.