What operating system are you using?
How is the drive formatted?
Have you checked the permissions on the drives ROOT. Folders under ROOT might have permission for your username to Read/Write but not the ROOT of the drive.
I'm using a MAC with the latest iOS and the drive is actually a second back up drive. It works for my regular Time Machine backup. I can even copy files into to. I just have trouble with LR recognizing the drive, unless I'm adding a folder into an existing folder already. The permissions is the same as my first back up drive. The first backup drive has no problem with LR adding folders