Where to store (personal) files? What is the best place in the Android (4.4 or similar) file system to save some
documents (.pdf, personal docs, other files, music, ...)? In other words, is
there a specific folder where one can create a home directory and put in files
without having to fear that it is wiped out by (updates of) Android?
Note that this is not guranteed to be the case with every update, and may vary from device to device. However, in every case I'm familiar with, this has held true.

OTA/Automatic updates (and many but not all ROMs installed with a custom Recovery) will generally ignore anything inside the /sdcard/ directory. I keep everything in there and whenever I switch ROMs or update, that directory stays safe as long as whatever I'm installing doesn't require a factory reset.

Devices with no physical SD Card slot such as Nexus, and Kindle Fires have treated that sort of as the default /home/user/ directory. Downloads from Chrome go to /sdcard/Download/ by default. Music gets downloaded to /sdcard/Music/ etc...

Devices with a physical SD Card slot seem to use that as the mount point for an inserted card. You can use it if you have a card inserted - otherwise, it's not available.


Some devices will be different. As Izzy noted, the external SD card can have a different default mountpoint such as /sdcard/external_sd/ or something with /extSD/. It could realistically be any point that the manufacturer or ROM builder decides on.


Having said that, it's always a good idea to keep your data backed up. I haven't had any emergencies, and my data in the directory has never been wiped when I wasn't expecting it, but it's always best to be safe.