Try right clicking on it and select Properties. Then click on the Security tab. In the Group or user names area, click on the SYSTEM account in the list and in the window below that make sure Full Control is checked. Do the same for the Administrator account. If Full Control is not checked, then select that account again and click the Edit button and check Full Control in the list. and then apply your changes and exit the window. Reboot your PC and see if you can access the drive.
If none of this works, I suggest plugging the drive into a different PC since it is external, and see if you can get your data off it copied to a different location. Then I recommend reformatting the drive and plugging it back into the original PC you were having the issue with and see if you can access it. If you can, copy you data back onto it from the backup location on the other PC and all should be well.
If not, well... I tried... 
