with the HD removed, can you still view the cameras? If so I suspect the HD is shot. Mine kept doing a similar thing. I could still see all the cams, but trying to do anything with the HD and I got the 'please wait' message. In spite of these HD's being made for the units, they still run hot. I have changed the position of mine, plus raised it so that air can circulate under it.
Formatting on your PC and it probably won't work in the unit. They use some type of Linux system I believe. Did the HD work in the PC?