Francopacks, did you ever manage to solve this?
I've got this exact same problem with my LS-WVL. Adding a second disk is pretty bog standard, so I doubt that updating the firmware and/or resetting to factory defaults is the solution. There must be something else that can be done, surely?
BTW, not sure about you Francopacks, but I'm using Shock's 1.54 mod1 open firmware. Yes, I realise that this is non-standard, but I highly doubt that Shonk has changed the parts of the firmware that handle the HDDs; ergo, I would expect to be afflicted by this same problem if I were using the official Buffalo firmware.