Have you tried the web interface (my.pogoplug.com) to mount/attach the drive?

If you use the web interface and the PogoPlug detects no drives it should show a button to refresh the drive list. Look at the bottom left column in the main PogoPlug window (at my.pogoplug.com), do you see a flag that says "no drive dete..." (for "no drive detected on PogoPlug"). It's at the very bottom of the leftmost column (may be hard to see if not selected, since when not selected it may only appear as gray on a gray background). Select that button and you should see a window that allows you to scan for attached drives (window button says, "I attached a drive, refresh the screen.").

I believe that the way the PogoPlug works is that if a drive is dismounted you need to completely detach the drive from the PogoPlug and then reattach and power the drive. Then use the above method to refresh the drive list. I don't know why my PogoPlug seems to work this way, it could be a bug or a safety measure (safety since there is no real UI for the device itself and you shouldn't be disconnecting and connecting drives without using the PogoPlug client software on your PC/Mac).