If the format is ext4, it should be available in 4.6.8 / 4.7.0 and 4.6.6-op24, I tested on my router and works ok.
Please check the format first.
MT6000 with 4.6.6-op24 and 4.6.8 / 4.7.0
USB disk: ext4
Access on the MacBook: smb://192.168.8.1/disk1_part1
