Smart previews are created in the same folder that contains your catalog. If you want smart previews on your main hard drive then you should put your catalog and your previews folder on the main drive as well. Smart previews are useless without the catalog. So if you disconnect the external hard drive you have no way of accessing and modifying the smart previews if they are the only thing on your main hard drive.
Smart previews associated to a particular catalog stays to the same place where you have the catalog.
As in the absence of the original images the catalog would allow you to use the smart previews to do further editing.
Do you want to put the catalog and the smart previews at different locations?
You can leave the original images and catalog both on external drive to save space.
Bedankt Sarika and JimHess
If I understand it correctly I have to put my catalog on my internal HD
But is is wise to leave the original photos on my external disk and put the catalog on my internal HD.
I thought its always advised to leave those two together?
The catalog can be anywhere that you want. By default, Lightroom creates the original catalog on the internal hard drive. That's where I keep mine. The only time the catalog needs to be on the external drive is if you're going to move that drive from one computer to another. Otherwise, there is no advantage or disadvantage to having the catalog on the external hard drive.