I want to give it a try on SMART PREVIEWS (i know i cant merge panos or hdr with sp) BUT i want to check how my mac performs with smart previews in relation with my RAM.
I guess I don't see the point of doing this. It won't tell you anything about RAM, which is a problem when you create panos or hdr photos, but not otherwise in Lightroom, since you have 16GB. The testing seems completely unrelated to the original problem you stated to justify some sort of testing.
The problem is that after i create 10 photos just to check (i never before use smart p.) i try to EJECT the disc which my LR CAT is stored and says that i cant because Lightroom uses that disc.Yes i know but thats the point of smart preview,WORK OFFLINE and sync after.
Smart previews work when the drive that has the photos is offline. It does not work (and Lightroom will not work) if the drive that has the catalog is offline. If the catalog and photos are on the same drive ... then you can't eject the drive.
If you really want to do some sort of testing of the smart previews (again, not sure what testing you are doing or why you are doing it), you can move or rename the original photos so that LR thinks the photos are missing, and it will use the Smart Previews. Or you can turn on the option in Lightroom that uses smart previews even if the original photos are present.
It would be necessary for the catalog and smart previews to be on your internal hard drive with the master images on the external hard drive in order for your plan to work. But I don't know that it would solve your issue. I'm working with a very modest older HP Pavilion computer, 8 GB RAM, "upgraded" from Windows 7 to Windows 10. I'm able to create panorama images using 21 24 MP raw images in three horizontal rows. It isn't lightning fast, takes about 5 min. to build the panorama but it works.
But the the main point of my message is that you will have to move the catalog and smart previews to the internal hard drive if you want to only use the smart previews when the EHD is not connected.