If you have not formatted the drive for use previously, you need to format it. However, if you have already done so and it is still flashing, chances are that the drive inside the external enclosure has gone bad and needs to be replaced. If you have stored any data on it that is irreplaceable (such as your only copy of family photos), then before you replace the drive (especially before sending it in for warranty service), you should consider how important that data is to you and whether you wish to spend the money on a reputable data recovery service such as Ontrack (found at
Kroll Ontrack recognized leader in legal technologies and data recovery...). Be aware that such services tend to cost around $1000. Fortunately, if you tell the service what data is most important to you and they are unable to get that (even if they can get everything else), many services will not charge you for said recovery but will provide you with what they were able to recover.
I hope this helps. Let me know if I can assist you further.