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It is possible you have already-imported items that were not copied to the library storage location. If so they would have to be consolidated to the selected library storage location. See MacBreak Studio "Media Management in Final Cut Pro X 10.1.2 - Guidelines & Operations":

 
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The setting in Preferences sets the default behavior for new Libraries, but it is possible to change the setting on a per Library basis, so I would check that, the posted video above covers how-to in detail.

Before I delete anything that I've imported (or think I've imported) into a managed library, I always run the consolidate media command to be 100% certain.
 
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