Synchronizing 1 t hdd with engine prime failed at 300 gb

Ok . I’m using Windows 10. Original tracks not in bibliothèque on Disk C but on external HDD. I switch the external HDD for MCX 8000 on pc and run ENGINE PRIME. Doing some crates and synchronizing for add on the external for MCX. Synchronizing failed when HDD is about 300 GB downloading new tracks. What this for ??? And what will happen if I buy PRIME 4 with internal HDD 2 T ???