Mounting external sd-card on rooted Android 7.1 tablet fails, but only if partition not empty
Does sdcard mount Android storage setting?

Run blkid command, see what say about partition.

Also what is mountpoint? I.E. /mnt/sd-ext

If you call mount like this

# mount -t ext4 /dev/block/mmcblk1p1 mountpoint


You have to be in same directory as mountpoint