My Xiaomi Redmi Note 4 bought in 2017 was driven by Android version 6.0.1 and MIUI version MIUI Global 8.2 /Stable.
In the past I used the smartphone run by Android 4.1. As soon as I got the new smartphone I had to deal with some unexpected troubles that took me significant time to overcome. I hope my experience may be useful for other happy owners of this phone.
The surprises for the novice to overcome
First, I am a heavy user of GPS, in particular in recording gps tracks. I did not suspect to encounter any problem here. I use regularly Locus application. After installing Locus app I found out that the gps signal is acquired really fast. In fact, it is a matter of a few seconds. That was a pleasant discovery. The worse was, however, to come. On my way from home to work by bike I started GPS recording in firm belief that after arriving to my destination I will have the all track recorded without any problems. To my surprise, I had just 2 points recorded, the starting and the end.
I spent several day investigating and searching on Internet the cause of the trouble. In vain. Then I visited the vendor of the smartphone and got none the viser.
My searching discovered that the problem lies in the fact that as soon as the display goes off or the smartphone is turned off, the Locus programme stops recording at once. Then I tried several other gps recorders (apps). The result was annoyingly the same. All the apps did not work permanently. That was a new problem for me. The older Android version worked without any hitch regardless of the on/off phone status.
After many trials I discovered that the culprit is the unfortunate default setting in Security/Battery/App battery app. In Locus programme, I had to change Background settings to No restrictions and set Background location to "Allow". The crucial setting was "Allow" that enables apps to use location services in the background.
This setting must be done individually also for any other gps recording app.
In the end this solved the mysterious problem.
First I installed an additional SD card. Then I connected the Xiaomi to my PC (OS Windows XP) but was unable to figure out how to transfer files between PC and my smartphone by using a cable with USB port. As I have two OS installed, then I tried Ubuntu OS. The result was negative again. After many trials and errors I installed an app called USB setting. Then I discovered that when I swipe down, touch USB for charging icon, then switch to transfer files (MTP), suddenly, I was able to see both Android internal and external SD card and move files between two devices.