According to

http://fr.audiofanzine.com/synthe-numerique/yamaha/montage-76/news/a.play,n.31466.html

the Montage has "4 GB of waveform storage" ("Mémoire de 4 GB pour les formes d’onde"). If this refers to the flash size for libraries, it would be a bit disappointing. But I wonder where they got the information from and if it is reliable.