If you have used the drive on a Windows system before/you are using it for the first time on your Mac, there is a chance that your drive needs to be reformatted to the correct MacOs-supported file system. You can do so by following the steps below:
Reformatting required for Mac OS Compatibility:
Before doing that I would advise you to connect the drive to another computer to back up your data as reformatting the drive is data destructive.