D7000- 30 minute exposure limit?

I set up last night to shoot some star trails, but my D7K stopped the exposure after only 30 minutes (1799.7 sec). I had intended to leave the shutter open for 2 hours. Here are the pertinent settings:

Manual mode: ISO 100, 18mm, VR off, F5, manual focus, shutter "--"

Release mode: Wireless shutter release on the dial and in 'mirror up' mode selected through the shooting menu.

The frame was focused in live view mode and I used the wireless shutter release to start the exposure.

I can't figure out why the exposure stopped after only 30 minutes, because I did not press the remote to stop the exposure. The only thing I can think of is because the release mode dial was in remote control mode, the slowest shutter speed displayed as "--". All the other release modes have the "bulb" option. I thought that both of these options are dependent upon the user to start and stop the exposure.

Is there a difference between "--" and "bulb" when it comes to leaving the shutter open as long as you want? On a side note, if you rotate the dial one more click past "bulb" the shutter speed displays "x250". What does "x250" mean? The manual does not elaborate.