How to fix this disk has 2 partions popup on external HD
It certainly looks like that disk was at some point partitioned. Open Disk Utility, click on the disk and look under the Partition panel. Disk Utility will show the current partition scheme, and whether you can repartition the disk without losing data. But back up your data onto another disk or your computer before trying that! 

In the unlikely event this is just a glitch, you should certainly run Disk Utility Repair!