When relink fails it doesn’t give you much info. However if you pick only one file (which failed previously) and relink that, it will give more specific info about why it failed. Try that, then examine the file metadata on the source and destination machines using MediaInfo, Invisor, etc.
The library contains metadata about each media file. During relink this is compared to the metadata read from the disk file being relinked. Unfortunately I don’t know if an easy way to examine the stored metadata, except to export an XML, open it in TextEdit and search for the filename. That will lead to something like this:
<asset id="r28" name="C0099_28672" uid="F51E6A6ED48F2F90D9A86998A9001A05" src="file:///Volumes/TBay8TB_SSD_True/CMA/CMA060518/HarleyA6500/C0099_28672.MP4" start="0s" duration="6186180/60000s" hasVideo="1" format="r27" hasAudio="1" audioSources="1" audioChannels="2" audioRate="48000">
If you export an XML on your destination machine, then for a problem file compare the metadata within the XML from both source and destination XMLs, this will show whether the file metadata (from FCPX’s viewpoint) is the same or not. If it’s not the same the question is why not. In that case the next step is examine the source and destination media files themselves using MediaInfo, Invisor, etc. If they are the same, the question is why is relink failing.
BTW the latest version of Final Cut Library Manager can produce a .csv list of all media files used in a library, including pathname, but it doesn’t list metadata. It would be nice enhancement if it did.
If you are using external proxies, ie proxies generated by FCPX but stored outside the library, relink will be unreliable in that case. That is a known problem I filed long ago.
Are both media sets on both machines on HFS+ partitions? If one is a portable exFAT hard drive, maybe that’s a factor.