An error has occurred during REBUILD and when I reopened the MCG Manager most of songs are gone. What did happen to the songs?
An unknown error might had occurred during the rebuild, and if most of songs were deleted, go to TRASH and there should be a folder called _BUILD and which contains those deleted songs files. Right click on folder and choose to restore. Then go back to song library folder inside USB Hard Drive, and double click to open _BUILD folder. Copy the songs files and paste those into _Songs folder.