I detailed one issue I ran into here on this post: https://android.stackexchange.com/questions/124575/mount-an-ext4-formatted-sdcard-on-a-stock-lolipop-5-1-rom

I detailed one issue I ran into here on this post: https://android.stackexchange.com/questions/124575/mount-an-ext4-formatted-sdcard-on-a-stock-lolipop-5-1-rom-rooted https://android.stackexchange.com/questions/124575/mount-an-... Looks like some comments were added three years later. The entire mounting system between the Linux command-line/system interface and the Android UI is terrible disconnected and the way to trigger events in one changes in every single release. Having just a knowledge of Linux is not going to help you debug problems with Android. Android also does a lot of crazy/non-standard SELinux isolation stuff. I do a longer/more complete rant on Android from a few years back here: https://battlepenguin.com/tech/android-fragmentation/ https://battlepenguin.com/tech/android-fragmentation/