Hi granada29, Thank you for your reply. I finally got it working earlier today by downloading the Big Sur installer software and installing it on the SSD. I ran the Terminal command "softwareupdate --fetch-full-installer" and after it completed it asked me where I wanted to install it. I chose my external SSD Drive and at the end I could finally see under "System Preferences" , "Startup Disk" both options: My Mac Mini HD and the External SSD. By selecting the SSD as a boot option, everything worked great! I hope this might be helpful to others having the same issue with mine. Take care, Nikos

Hi andyl_, Thank you for your valuable comments. I did what you suggested and it looks and works great. Take care

Hi DarrenCNDR, Thank you for the feedback. I guess I should invest in SwiftUI if I want to seriously develop a WatchOS app. I appreciate your reply. Take care, Nikos

Hi Claude31, Thank you for your reply. Yes, it is unfortunate. I did some research before posting my question because I was not sure if I am missing something. I guess Apple is promoting SwiftUI for WatchOS development. I just have to comply :( Take care

Hi darkpaw, I appreciate your informative reply. I can see why SwiftUI is the preferable choice for WatchOS app. I am already spending time to learn. Thank you for the link as well. Take care :)