The app uses a number of different methods to list, read, and write files. Each of these are detailed below.
This is the original and default method of accessing files. The app simply scans the underlying file system for available files and folders. Likely storage locations are automatically found, or can be added using the "Choose Folders..." option in the menu. Being able to list and read a folder does not necessarily mean it can be written, but the app will always try if requested.
If your device is rooted, you can set root access in the settings. This works exactly the same as normal access, except that the user will have access to all the folders on the device.
By default, only read access is set - write access will be done via normal access. This is to prevent the user accidentally damaging the system. However, you can set write access to use root as well, which allows you to delete and modify read-only areas of the device.
On Android KitKat (Android 4.4), writing to external devices such as SD cards is not possible unless you have root access. The app will still use normal access, but writing to external devices is blocked by Android. You will need to you the system to modify files, upgrade to Lollipop (Android 5.0), or use root access.
In Android Lollipop (Android 5.0), a new method of write access was introduced, which allows writing to external devices. If the user tries to modify a file and Android blocks it, the app will ask the user to grant permission to edit the device.
A system window will be shown, and the user needs to choose the folder to grant access. To ensure the right folder is chosen, you should grant access to the entire external device. You may need to show the SD card before being able to select it.
Once permission s granted, the app will have permanent access to that SD card so further modifications can be made without permission being explicitly granted again.
The images below demonstrate how to grant permission to an SD card. It assumes that the SD card is NOT shown - if it is the process is simpler. Hard drives and other external devices are done in the same way.
In Android Nougat (Android 7.0), the previous method has been simplified. Instead of the user having the choose the folder to grant access, the app can ask for access to a specific device. Once again, this will occur if the user tries to modify an external storage device. But instead of a system window requiring a folder choice, the app will simply show a popup asking for a permission to a specific device. As before, if granted, the app will continue to have access to the device and it does need to be asked for again.
Once again, this has been made more complicated from Android Q (Android 10.0). This has moved back to a process similar to how it was for Android Lollipop, but has been made a little easier because the app can ensure that the correct folder is initially requested.