Thanks for the solution. But not always has an external memory device. I want to store data on the device in order to find out whether the previously run this app or not. The application for the catering, the seller wants to give a discount to those who first makes an order through the app.When an app is uninstalled all the related data is also removed, however, any files created might stay if for example were created on DirRootExternal.
It is a very bad practice to leave "data forever" taking unnecessary space on the device, besides, it could also be a privacy/security issue depending on what kind of data has been saved, in other words, when the app is gone all related data should be also gone.
Yes I understood you. I'll look for then on the device unique values, store them in a database on the website and compare values. I will try to use the name in the google account. Like this: Determination of the mail address of the deviceThat's not a solution, those files can be easily deleted with a file manager, besides, all devices have external memory (don't be confused with external as a USB dongle)