I have had this happen quite a few times, but I have no idea what actually triggers the problem. I have found no fix that doesn't involve removing it from the Passion+ and connecting it to a PC.

Once plugged into the PC, making absolutely any change to the HDD resolves the problem so far as the Passion+ is concerned. Adding a file, deleting a file, renaming a file or simply renaming the HDD all fix the problem - any write to the drive works. The renaming of the HDD is the quicker option and the Passion+ doesn't seem to mind what the drive is called.