You could try these suggestions from the pogoplug knowledge base. The latter resolved my problem.

http://pogoplug.zendesk.com/entries/221759-my-hard-drive-is-not-available

http://support.pogoplug.com/entries/221766-i-tried-a-clean-restart-and-my-drive-is-still-not-available

Good luck and enjoy your pogoplug

Remember, its not what life does to you, but what you do with what life does to you, that matters.