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Sorry, the reason that won't work is the unique ID of each track will now be different, and thus not a match for the tracks on the export USB drive... even if it's the EXACT same song from the exact same location on your drive as when you originally exported it. When you move to a new computer, you should move your database as well. An export drive can't (currently) be used to rebuild or import anything other than the music itself, everything else must be done within rekordbox from scratch.
The solution is coming down the road, and it's a limitation of the current database system. We hope to see it in a future version, but no timeline is available yet.
If that USB drive was exported from that collection, what you want to do is right-click on the USB drive and select Update Collection - that will transfer back any changes you made to grids, memory points, hot cues, playlists, etc.