The disc always floats 1 foot off the ground.
The rules say:
You create a slightly concave, circular plane of force that floats a foot off the ground and can carry what you lay upon it. The disk is 3 feet in diameter and 1 inch deep at its center. It remains stationary unless you move more than 5 squares away from it, in which case it moves with your base speed once per round until it is within 5 squares of you. You can command the disk to move up to your speed as a move action. If you are more than 5 squares from the disk for 2 consecutive rounds, the disk disappears, dropping whatever it was carrying. DDI (emphasis added)
- If you fly up, you will eventually move more than 5 squares away from it.
- At that point, it will move as close as it can to you.
- But since it remains only one foot off the ground, it will just hover directly below you.
- If you remain in the air more than 5 squares away from it for 2 consecutive rounds, the disk goes away. Poof.
The rules do not say what happens if you summon a disc while already flying.
There are multiple interpretations of the first sentence quoted above.
- The disc is created below you at a point one foot off the ground.
- The disc is created nearby but immediately falls until it is one foot off the ground.
- The disc cannot be created because it must always float a foot off the ground.
Frankly, I'd go with #3.