I have analyzed my tracks numerous times and still some playlists are not loading as analyzed on my prime 2. This has only just started being an issue for me and not sure why.
What format is the USB ExFat or Fat32
Fat32 as it is a 64GB USB. Some playlists work fine, others analyze on my prime 2 even though I have ran all my music through Engine.
The DB file may have reached the max size FAT32 allows due to analysis data.
Try a exFAT format for the USB. It does not have that limitation.
Basically every time you analyse a track that data is stored in the DB file