Mounting external sd-card on rooted Android 7.1 tablet fails, but only if partition not empty

Edit: I have tried mounting the sd-card with an empty ext4-partition, and it worked just fine, but after I ran debootstrap on the sd-card, the problem reappeared with identical symptoms. ---- I want to mount an ext4-formatted sd-card, but # mount -t ext4 /dev/block/mmcblk1p1 mountpoint returns mount: mounting /dev/block/mmcblk1p1 on mountpoint/ failed: Invalid argument So far I have: * made sure the sd-card itself is OK (mounts fine on my debian laptop) * made sure the block device is the right one * replaced mount with busybox mount , which is a different binary, but gives the same error The mountpoint exists, and the kernel supports ext4. My device is a rooted Samsung Galaxy Tab S2 running a Lineage OS 14.1 ROM (Android 7.1), SELinux is set to permissive. ---- Any pointers are appreciated!