It sounds to me like you possibly have a different username on the new computer and it can't find the existing tracks. If you use the relocate feature it should be able to find them once you find the first track.
I recently had a drink dropped on my old laptop, but the hard drive was saved. I reinstalled itunes and rekordbox onto my new computer, followed the steps of unhiding your library and copying the old databases.. Everything seemed fine, all the files and playlists are there, but when i went to play them the yellow warning came up. After trying to relocate all missing Media, it said 25000 (About 1/2) songs were missing. Any idea of what went wrong? Re-importing that many songs one by one isn't my idea of a good time. Also, unless the original source files are back in the library I can't re-import them from my USb stick which has all the tracks....
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It sounds to me like you possibly have a different username on the new computer and it can't find the existing tracks. If you use the relocate feature it should be able to find them once you find the first track.
If you indeed have a new username you might find this helping:
http://forums.pioneerdj.com/entries/52374367-Problems-with-rekordbox-synching-and-license