If you have experienced issues where you were unable to copy your songs/mp3 files to your iPhone device using drag and drop feature, here's what you need to do.

Open iTunes and connect your iPhone. In the left pane click on the iPhone and make sure that Summary tab is selected.

If you are like me, an iPhone or iPad user and have the need to restore the software on your device, you might receive the following error:

The iPhone could not be restored. An unknown error occurred (1611). 

You may also receive this error with error code (1604) and (1609). If you find this solution useful and had a different error code, please post your comment below.

Solution

I have tried various sources on the net, but I haven't found anything that would solve my issue, although the solution was fairly simple.