FCPX creating a duplicate library in Movies?

I've had a pretty good search but not been able to find an answer to this, and it's rather baffling me.

I want my FCPX Libraries on a non-system drive (Mac Pro 5.1, with a rather full SSD system drive but tons of mechanical storage). By default FCPX creates a mylibrary.fcpbundle package in Users/applemac/Movies, and it's this that I copied to a storage drive. I delete the original package from the Movies folder. (I didn't empty trash.)

I start FCPX, hold alt, and locate on the storage drive the library I want to use. Watching Finder at the same time I see another fcpbundle package appear in the Movies folder, ie on the system drive. It's 12GB and contains all the media from my libraries!

I've done it several times and it's consistent. FCPX runs fine, apparently referencing the library on the storage drive. Well, fine UNTIL I empty the trash. Then, projects full or red regions of "Missing file" death.

Is there something fundamental I'm not understanding about FCPX file management? If anyone can fill in a few spaces here that would be great.

I hope that's all relatively clear!

Rather than using the Option key, after you move the library to the trash, simply double the library on the external drive to launch it.

Please confirm that your library is using managed media and not external media. What software version and what version of the OS is this?