I have used the Bend Will shout on Sahrotaar. He asks me to get on board, then nothing. I've tried fighting him. reloading lots of times. The Quest arrow is above his head and I've approached him from every angle, but to no avail. I need this fix!!

Have you tried earlier saves? Are you using mods? ANy that affect dragons?

Do you have this: skyrim\Data\meshes\actors\dragon\character assets\skeleton.nif

Try to delete skeleton.nif

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Also you can try this:

Press the Tilde Button on your keyboard (~), type tcl. Close console (press the Tilde Button again).

This will allow you to fly or float to the top of Miraks Tower. Once there, open console and type tcl again. Close the console.

This returns you to clip mode.

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Of course as Jasper said, it could be mod related as well. !!!