1. Prepare an USB disk with no partition. For example, using fdisk to delete all partitions and you should only see only one device, e.g. /dev/sda.
2. Insert the USB disk on i.MX6 device. Now, everything is fine. You can access the contents in USB disk correctly.
3. Remove the disk and type df command to show mount information. You can see /mnt/udisk is still mounted.
4. Insert the USB disk again. Mount procedure is invalid and you cannot access the USB disk anymore.
The only way to recover this symptom is to umount /mnt/udisk.
Please help to investigate this issue. Thank you.