# Please do not connect yet
ls /dev/block > /sdcard/mmcr-be
# Please connect USB device
ls /dev/block > /sdcard/mmcr-af
# Compare
diff /sdcard/mmcr-*
When you run diff, there is a line that starts with -
, so keep an eye on it.
Do you have multiple partitions? Then try doing this.
fdisk -l /dev/block/YOURDEVICE(without number)
mkdir /sdcard/mntUSB
mount /dev/block/YOURDEVICE /sdcard/mntUSB
Hooray!
In the case of TWRP, the USB device of multiple partition cannot be mounted automatically, so I wrote it here.