FCPX Storage

You seem to have a couple of points of confusion.

When you delete the Generated Library Files you are only deleting the render files which help with the playback whilst you are editing. These files are massive . . . possibly 10 times as big as your normal media and are not necessary once you have finished editing. Should you want to return to the project at a later date FCP will re-generate them for you.

Secondly, you can't put the FCP app on your external (other than for storing a spare copy). You must use FCP on your internal drive but as you have been told, the Library which is completely separate, can live on the external.

You don't have to do all that copying. Just keep everything ion the external drive. The default location for the library called Untitled is in the user's Movies folder. If you're already using that library just copy it to the external, or create a new library File>New>Library and save it to there external drive. You can either save your media inside the library, or leave it on the drive in a folder outside the library.

Yes, you can copy the library.

To avoid taking a long time and wasting space, first open the library in FCP, select it in the browser sidebar and do File->Delete Generated Library Files.

Check everything.

Then quit FCP and drag the library to the external drive in the Finder.

Also: if the external is fast enough you can also use the library in FCP right from the external drive.

Make sure that Final Cut Pro has “Full Disk Access” in System Preferences->Security & Privacy->Privacy

You can store your media either inside or outside the library bundle. The default behavior is to store it inside the bundle, similar to the way the Photos library works. To check where the media is select the library in the application, and in the Inspector you will see the library properties. You can change where the media is stored, or leave it inside the library.

For an excellent introduction to FCP start with Steve Martin’s 60 minute intro https://www.youtube.com/watch?v=HqUP7Zgeuck. For a more in depth introduction try the IzzyVideo tutorials. The version is a bit dated but the principles are the same. 

What computer, what OS, and what FCP version are you using?

What happens when you click the option to Update? If your version of FCP is newer than the one used to create the library, it likely needs to be updated, which the application does. You should not need to open a backup.

In a Library of 100GB it is quite possible that 90GB could be the generated render/optimised files meaning that your basic Library that you need to save is only 10GB.

It will take a while to re-render . . . anything from a few minutes to an hour or so depending on the size and complexity of your project but that is better than clogging up your hard drives on the off-chance you may re-visit the project in a few years!

One other point . . . there is no point in saving an old Library unless you are fairly certain you will want to re-edit the video.

All you really need to save is the finished video together with the original footage on the tapes or SD cards. You will then have everything you need to start a new version of the project should the need arise.

I am a belt and braces guy so not only do I keep the original tapes/SD cards, I also make copies of them on several cheap USB 3.0 HDDs. An hour of 1080p video is only around 8GB depending on the compression of your camera so a cheap 2TB HDD can hold about 240 hours of footage.

There should be a database backup for the library in the Final Cut Backups folder in Movies. What happens when you open that from inside FCP?

You must have started using FCP with 10.6.1 or earlier.