Did you read the advice link posted above?
I doubt it advised you to open any project on an external drive.
If you have multiple projects, copy each .scriv folder to Documents on your C drive, into a dedicated sub-folder called Scrivener (after all why make life complicated).
Click into each of the .scriv folders and launch your projects from their respective .scrivx files.
As far as licensing goes, yes the download of Scrivener onto your new machine will ask you to enter the number, whatever it’s called. If you’ve lost it, contact L&L customer support.
In future, access your programs from the icon. Scrivener will remember where they are.