A common problem is an external drive loosing its partition table information. As a result users get an message which asks them “Do you want to format this drive”.
If the drive is healthy there are several easy ways to recover your data.
Here is a very useful piece of free software which can help you, recover your files: http://www.cgsecurity.org/wiki/TestDisk_Download
The software is command line based. You should carefully read the instructions and follow them as the program executes.
Please consider donating to the contributors of the software.