That's normal for an iMovie library to do backups, particularly if not opened for awhile, or perhaps first opened with a new iMovie version or new OS. A 1.3 TB library is massive and would take quite a while to back up. Hence the spinning wheel. It would back up to your Mac's hard drive. However, it would be much smaller in size than 1.3TB because the media itself is not being backed up. The back up would refer to the media in the original library. If you wait patiently, iMovie should open after the backups are done.
Here's where the backup will go:
Go/Home/Library/Containers/com.apple.iMovieApp/Data/Library/Caches/iMovieBackups
You should see previous backups of your various libraries there. Note that they are smaller in size than the original library.
As for having to go to the Finder to open a library, you don't need to do that if you hold down the Option key while opening iMovie, and then selecting from the box that appears the library that you want to open. iMovie will open in that library. The next time that you open iMovie in the regular way, it will open in the library last used.
As I mentioned above, a 1.3 TB library is quite massive. Performance speed will be much slower, as you are finding with the backups. Also, if your external drive goes south, or the library gets corrupted, as can happen, you would be in danger of losing all of your projects and media, unless you keep a Time Machine Backup or a manually updated copy of the library on a separate external drive.
Everybody has their own way of organizing. I never let my libraries get larger than 60 GB. I back them up on separate hard drives. I also keep all of my media separately, and doubly, backed up in Finder folders on separate external drives. I create new iMovie libraries each year and name them iMovie library Year 2018, iMovie Library Year 2019, etc. I find that that keeps the libraries more responsive and easier to copy and back up without waiting forever. If you need to access projects and media in another library you can always copy them over to the currently used library.
-- Rich
