Drive Cannot be Modified

I have a Western Digital Passport with about 405GB data on it.  My Mac HD has 118GB available on it.

I recently tried to move a file to my external and I got an error message that the drive cannot be modified.  I have used this drive numerous times in the past on this same computer with no issues.  I was hoping to not have to reformat my External.  I DON’T/CAN’T lose all of my data.  Any suggestions?  

Hi there, what if you try to repair the drive using disk utility? (Go> Utilities> Disk Utility)

If you don’t want to lose your data, then don’t leave it alone on your Passport as it can stop working tomorrow, just so you know… ALWAYS keep your data on at least 2 different places, otherwise you don’t have a backup…